OBJECTIVE: To describe the evolution of the HIV-1 epidemic among women attending antenatal clinics in four different geographic areas within two rural regions with differing intervention intensity in south-west Tanzania. DESIGN AND METHODS: Age-specific trends in HIV-1 seroprevalence among antenatal clinic attendees in four distinct geographic areas of Mbeya region between 1988 and 2000 and in one area of Rukwa region between 1991 and 1999 were analysed and compared. In Mbeya region a comprehensive AIDS control programme has been implemented since 1988. Indicators measuring behavioural change and the attendance rate of patients with sexually transmitted diseases (STDs) were monitored through the established routine system, complemented by two Knowledge, Attitude and Practise (KAP) surveys in 1995 and 1999. RESULTS: Data showed an increasing HIV-1 prevalence from 1988 up to 1994/95 for women in the age group 15-24 years across all strata. Between 1994/95 and 2000 the prevalence declined significantly in all strata for this age group in Mbeya region with diverse patterns in spread of the infection accompanied by a significant decrease in the positive syphilis serology, high rate of condom use, significant delay of age for the start of sexual activity of primary school pupils and a high treatment rate for STDs. The increasing trend of the HIV-1 prevalence in Rukwa region continued. CONCLUSION: Declining trends of HIV-1 prevalence among women aged 15-24 years may correspond to a reduced incidence partially attributable to changes in behaviour and reduction of a biological factor influencing HIV-1 transmission to which the implemented programme could contribute.