The objective of this study was to describe trends over time in HIV prevalence, sexually transmitted infections (STIs) and sexual behaviour among women in Moshi urban, Tanzania. Two cross-sectional studies were conducted in 1999 and in 2002-04 among women attending three primary health-care clinics. They were interviewed and screened for HIV and STIs. There was a significant decrease in HIV prevalence (11.5-6.9%). The decline was greatest among women aged 15-24 years. Syphilis, trichomoniasis, bacterial vaginosis, genital ulcers and reported STI symptoms also decreased significantly over the three-year inter-survey period. The proportion of women reporting casual sex decreased and knowledge of STI symptoms and health-care seeking behaviour improved. Herpes simplex virus type 2, genital warts, age at sexual debut, age at first pregnancy and condom use remained unchanged. In conclusion, decline in curable STIs and casual sex partners may partly explain the observed decline in HIV seroprevalence. Both STIs and sexual behaviour should be monitored in HIV sentinel surveillance. There remains a gap between knowledge of preventive behaviour and actual preventive practices