Prevalence and incidence of HIV-1 infection among persons attending two STD clinics in Pune between May 1993 and October 1995 are reported. On screening 5321 persons, the overall prevalence of HIV-1 infection was found to be 21.2 per cent, being higher in females (32.3%) than in males (19.3%). Analysis of behavioural and biological factors showed that old age, sex work, lifetime number of sexual partners, receptive anal sex, lack of circumcision, genital diseases and lack of formal education were related to a higher HIV-1 seroprevalence. The observed incidence rate of 10.2 per cent per year was very high, much higher in women than in men (14.2% and 9.5% per year respectively) and over three times higher among the sex workers. Females in sex work, males having recent contacts with female sex workers (FSWs) and living away from the family and persons with previous or present genital diseases had a higher risk of seroconversion. Condom usage was shown to have a protective effect in seroprevalence and seroincidence analysis. With limited available resources and lack of a suitable vaccine or a drug, long-term prevention policy of creating awareness in the community must be supplemented by strengthening STD control measures and promotion of condom use and safe sex. Factors related to availability and utilization of condoms must be carefully investigated.