The Swiss HIV Cohort Study (SHCS) is a prospective multicentre study of HIV-infected adults. Participants have been followed up at six-monthly intervals since 1988. The purpose of the present study was to examine sex differences in developing AIDS-defining events and in survival among participants of the SHCS (1042 women, 1507 men). A statistically significant higher risk of Pneumocystis carinii pneumonia (PCP) was evident among women. This difference was particularly pronounced in 1989 (hazard ratio 2.11, p = 0.01). Other opportunistic events and survival showed no statistically significant sex differences. The results are compatible with slower introduction of PCP prophylaxis among women. A reason for this may be that women were less likely to be enrolled in clinical trials than men. This hypothesis will be examined in a further study.