This study investigates trends in AIDS knowledge and sexual behavior among men and women in urban Lusaka 1990 to 1998, and in all of Zambia, 1992 to 1998. Using data from representative surveys of urban Lusaka and of the country as a whole, population proportions were estimated to examine trends in knowledge and sexual risk behaviors. Differences in the estimated proportions between 1990 and 1998 were tested in Lusaka. In all Zambia, tests of difference were conducted between the earliest and latest years for which data were available for each indicator. A decline in premarital sexual activity was observed in urban Lusaka. In 1990, 50% of never married women reported no sexual experience, compared with 60% in 1998 (p= .003); among men, the figures were 38% and 53%, respectively (p< .001). Fewer women (1990, 8%; 1998, 2%;p< .001) and men (1990, 31%; 1998, 19%;p= .07) had extramarital partners. The bulk of change observed in urban Lusaka took place from 1990 to 1996; the changes in men's behavior observed between 1996 and 1998 were also observed in the national estimates for those years. National figures for other indicators from 1992 to 1998 were less encouraging. Apart from an increase in having ever used condoms, no change in women's sexual behavior was observed. Fewer men had premarital sex from 1996 to 1998 (1996, 64%; 1998, 46%;p< .001), but condom use with nonregular partners decreased among men (1996, 38%; 1998, 29%;p= .02). Prevention campaigns focused on education about AIDS and promoting safer sexual behavior appear to have made a difference in the early 1990s in Zambia. Findings from more recent years indicate that further change has stagnated. Renewed efforts are needed, particularly targeting condom use with nonregular partners.