Objectives: To investigate how age at first sexual intercourse is related to the reported circumstances and to determine how these corresponded to views in early adulthood about its timing. Design: Cross sectional study within a birth cohort using a questionnaire presented by computer. Setting: Dunedin, New Zealand in 1993-4. Subjects: 477 men and 458 women enrolled in the Dunedin Multidisciplinary Health and Development Study, comprising 92% of survivors of the cohort. Results: The median age at first intercourse was 17 years for men and 16 years for women. Only one man (0.2%) but 30 (7%) women reported being forced to have intercourse on the first occasion. For women, there were increasing rates of coercion with younger age at first intercourse. More men than women reported that they and their partner were equally willing (77% (316/413) v 53% (222/419)). Mutual willingness of both partners was greater for those who reported that it was also the first time for their partner. Timing of first intercourse was considered about right by 49% (200/411) of men and 38% (148/388) of women. Many women (54% (211/388) reported that they should have waited longer, and this rose to 70% (90/129) for women reporting intercourse before age 16. Conclusions: Most women regretted having sexual intercourse before age 16. First intercourse at younger ages is associated with risks that are shared unequally between men and women. This information is important to young people themselves. We questioned a cohort of young New Zealand adults about the circumstances of their first sexual intercourse and their views about the timing of first intercourse Median age at first intercourse was 17 years for men and 16 years for women. Being forced at first intercourse was commonly reported by women, especially those who experienced intercourse before age 14, and most women who had intercourse before age 16 reported that they should have waited longer More men than women said that both they and their partner were equally willing at first intercourse, and reporting of equal willingness was greater among those who reported that it was also the first sexual intercourse for their partner As well as pronounced gender differences in mutual willingness and retrospective views of appropriate timing, more women reported occurrence of sexually transmitted disease, especially among those who had intercourse before age 16