Dear Diary, : Adolescence: The coming of a new year is a traditional time to begin keeping a record of each day’s hopes and fears, triumphs and tragedies. The writer usually is a young girl, and the subject often is boys.

Jill Cohen is a senior in high school, far removed from her first post-pubescent anxieties. SATs have replaced boys--temporarily at least--on her priority list.

But on her nightstand remains a carry-over from her early teen-age years, her most precious tool for self-discovery. Her diary.

“It’s out there every day, where it’s always been,” said Cohen, of Granada Hills. “After all these years, it’s still important to me. I wouldn’t want anyone to read it.”

Teen-agers--especially girls--are known for their slavish devotion to daily diaries, cataloguing their experience in explicit detail before it escapes memory. And while VCRs, CDs and MTV continue to redefine the adolescent life style of the ‘80s and ‘90s, the day of the diary is far from over.

Indeed, companies have been making diaries for decades and do especially brisk business during the Christmas season. Although diaries do not necessarily contain 365 pages, one for each day of the year, the beginning of a new year frequently triggers an urge to keep a journal.

Michael DeMent, a spokesman for Hallmark Cards, which produces 12 kinds of diaries--varying mostly in graphic design--said they have grown in popularity. He said diaries, together with albums and other memory books, annually make up a $500-million industry. Each Hallmark diary is 188 pages long and doesn’t have dates and years so teen-agers won’t feel obligated to make daily entries.

“Teen-agers are registering the need to chronicle what’s going on with them and their families,” DeMent said. “We’re not sure why, but the number bought goes up each year.”

A diary allows Tamara Bell, 17, to create a new world. Bell, of Granada Hills, has poured her thoughts into her journal almost every day since ninth grade. Life in the San Fernando Valley isn’t always so rosy, she says, but the diary takes care of that. When she is writing, she transplants herself to a time and place where she imagines she is prettier and more sophisticated--where she always gets the guy of her dreams.

“It’s where I can play pretend,” Bell said. “I can live how I want to live. I have all these romances, and they’re always at the beach. In the diary, I just know the guy’s going to be mine. In real life, it’s ‘don’t you touch me at all.’ ”

Bell said she writes extensively about her family, especially her mother. “I write about how different we are and yet how similar we are.”

Bell’s cousin, Tonya Bell, 16, is also a diary die-hard. She releases her deepest, innermost thoughts about her newest crush, and what she wants to come from it. Like most teen-agers, she is understandably protective of her journal. She keeps it in a jewelry box and hides it in her bottom drawer under some boxes and clothes.

That’s not all. She has developed an intricate code to hide certain identities in case the book were to land in the hands of the wrong parties. Even close friends aren’t granted reading rights. “So even if someone finds it,” Tonya said, “they wouldn’t understand.”

All of the Hallmark diaries feature locks. “We feel teen-agers want to guard what’s there just like it’s their lockers at school,” DeMent said.

But do their thoughts become so guarded, so private, that the diary prevents them from confronting real problems with friends and family? Illisa Kessler, 17, of Granada Hills recalled a friend who became dangerously immersed in her written world. “She’d write in her book and not tell anyone anything. She doesn’t go out any more. We were the best of friends.”

Sharon Haber, a family counselor in Woodland Hills, said some teen-agers could get carried away but believes diaries are healthful outlets. “It might even encourage them to go to their family and friends because writing it down allows them to get more in touch with their feelings. And if they don’t go to their families, it’s not the diary that’s going to push them further away.”

Darla Livingston, 16, of Granada Hills said diaries have helped bring her family together. She said her mother, 54, has shown her the diary she put together when she was a teen-ager. “I only remember my mom being old. But she was actually like me. I can relate more to her now when she’s trying to help me with my problems.”

Yet even Livingston has her limits. She doesn’t let her mom know too much. The diary isn’t nearly as explicit as conversations with friends.

“I’ll talk with my mom about boys, but if I wrote it in the diary and she saw it,” Livingston said, “she’d burn the book or she’d burn me.”

Not surprisingly, diaries aren’t popular with teen-age males. DeMent said the company estimates that 90% of the items in the “social expression industry”--anything purchased in a card store--are bought by women. Glebb Gofin, 17, of Granada Hills seems to represent the general male perspective.

“You should be able to remember your feelings without writing them down,” Gofin said. “I remember my first love, my first kiss.”

(Replied a female teen-age friend: “That was a dream.”)

Over the years, as adolescence evolves into adulthood, the diary can serve as nostalgia, a reminder of junior high indiscretions.

“I look back now, and I can’t believe some of the things I wrote,” said Singe Jung, 18, of Northridge, a freshman at UC Riverside. “I’d write, ‘Oh, I love him. I want his kids.’ And then a few weeks later, I’d see what a jerk some guy was.”

Still, Jung writes in her diary every day, sometimes for a few hours. She finished one book--they typically run about 150-200 pages--in eight months, and is almost finished with another.

A recent entry:

“I met this new guy in psychology today. I forgot his name. I think it was Rob or Robert. He’s a psych-bio major. That’s a pretty interesting major. He was really nice but nice guys can’t be trusted.”