From 1995 to 2001, five rounds of sentinel surveillance were carried out for young women attending antenatal care clinics at four health centers in Addis Ababa, the capital city of Ethiopia, to monitor trends in the prevalence of HIV infection and syphilis. Serum samples were tested for antibodies to HIV (enzyme-linked immunosorbent assay and Western blotting) and antibodies to Treponema pallidum (T. pallidum hemagglutination assay and rapid plasma reagin test). Prevalence ratios for an increase in one calendar year were estimated using log-binomial models. Between 1995 and 2001, the prevalence of HIV infection among young women (age range, 15-24 years) attending antenatal care clinics in inner city health centers declined from 24.2% to 15.1% (prevalence ratio for an increase in one calendar year, 0.91; 95% confidence interval, 0.87-0.95). No change was observed for older age groups or in outer city health centers. The decline in the prevalence of active syphilis (T. pallidum hemagglutination assay and rapid plasma reagin testing positive for antibodies to T. pallidum) was more pronounced among and also restricted to the young age groups (age range, 15-24 years) in the inner city (from 7.6% in 1995 to 1.3% in 2001; prevalence ratio, 0.69; 95% confidence interval, 0.59-0.80). The declining trends in the prevalence of HIV infection and syphilis among young women attending antenatal care clinics in the inner city are encouraging, but these findings require confirmation in future years and for other population groups.