A study suggests that half of them contemplate killing or harming themselves, and four in 10 actually harm themselves at least once.
Almost a third harm themselves on more than one occasion.
The research also indicates that 17% - nearly one in five - display symptoms associated with post-traumatic stress disorder later in life.
As adults, some suffer flashbacks and nightmares linked with being bullied at school.
Gangs
The findings by Dr Ian Rivers, of the College of Ripon and York St John, were being presented to The British Psychological Society's Lesbian and Gay Section Conference at the University of Surrey on Tuesday.
They are the result of a three-year study into the impact bullying has on academic achievement and long-term mental health problems.
For the research, 190 lesbian, gay and bisexual people, with a current average age of 31, were questioned about their experiences at school.
The study indicates that the bullying they experienced was both long-term - lasting for periods of about five years - and systematic, and carried out by gangs rather than individuals.
Separate research looked at the experiences of bullied and non-bullied heterosexuals, as well as non-bullied lesbian, gay and bisexuals.
Findings suggest that as adults, those who had been bullied, whether gay or heterosexual, were found to show symptoms of depression, anxiety and internalised hostility.
'Section 28 stumbling block'
But Dr Rivers said these symptoms were displayed most strongly by lesbian, gay and bisexual people who had been bullied.
"I would suggest that there are fewer guidelines to censure bullying when it's over an issue of sexuality," he said.
Dr Rivers said he believed the repeal of Section 28, which prevents local authorities from promoting homosexuality, would help.
"Section 28 is a major stumbling block to discussing sexual orientation and homophobic bullying," he said.
Schools also needed to include sexual orientation clauses in their anti-bullying policies, and teachers needed to receive more training in pastoral care issues.
"So many children are misinformed when they could be informed, which would help to prevent bullying," he added.