Fertility rates in a population-based cohort of over 3500 women aged 15-49 years living in rural southwest Uganda are described and examined in relation to infection with HIV. Over a six-year follow-up period (1989/90 to 1995/6) the average general fertility rate was estimated as 199 births per thousand woman-years of observation (95 % confidence interval 191 to 207) with a total fertility rate of 6.2 births per woman. The overall prevalence of infection with HIV was 12 per cent and remained relatively stable during follow-up. With the exception of women aged 15-19 years, women who were not infected with HIV had higher fertility than HIV-infected women. The overall age-adjusted fertility rate in HIV-infected women was 0.74 of that of uninfected women (95% confidence interval 0.63 to 0.87, P<0.001) and this result was unaffected by additional adjustment for marital status. When combined with an overall HIV prevalence rate of 12 per cent, this corresponds to a three per cent reduction in fertility rates in the whole population. The lower fertility in HIV-positive women is unlikely to be explained by increased use of contraception, as use of modern contraceptive methods in rural Uganda is low and fewer than ten per cent of women are aware of their HIV-serostatus. More likely explanations are reduced sexual activity due to clinical symptoms associated with HIV infection or lower fertility associated with coexisting infections with other sexually transmitted diseases, such as syphilis. A reduction in fertility caused by HIV infection itself cannot be excluded. The implications of these findings for the use of antenatal clinic data to provide population estimates of HIV prevalence are discussed.