Background. This paper sought to examine the relationship between sexual orientation and mental health in a New Zealand birth cohort studied to age 25 years.

Method. The analysis is based on a sample of 967 participants (469 males; 498 females) in the Christchurch Health and Development Study. As part of this study information was gathered on: (a) measures of sexual orientation, same-sex behaviour and sexual attraction obtained at ages 21 and 25 years; (b) measures of mental disorders and suicidal behaviours over the interval 21–25 years; (c) measures of childhood and family background.

Results. Latent class analysis was used to combine indicators of sexual orientation, same sex behaviour and attraction to form an empirically based classification of sexual orientation. The best-fitting model classified the sample into three groups: exclusively heterosexual orientation (87·6%); predominantly heterosexual but with same-sex inclinations or experience (9·6%); predominantly homosexual (2·8%). Proportionately more women than men were classified as predominantly heterosexual (14·2% v. 4·8% respectively) or predominantly homosexual (3·9% v. 1·5% respectively). Cohort members with a predominantly homosexual orientation had rates of mental disorder and suicidal behaviours that were between 1·5 and 12 times higher than for those with an exclusively heterosexual orientation. These associations persisted after adjustment for confounding. The associations between sexual orientation and mental health were more marked for males than females.

Conclusions. The findings suggest a continuum of sexual preferences amongst young adults. Variations in sexual orientation were clearly associated with mental health. These associations tended to be stronger for males.