Several studies in sub-Saharan Africa have reported that HIV prevalence in young women is higher than in young men. We used data from Kenya HIV sentinel surveillance conducted from 1990 to 2001 among sexually transmitted disease (STD) patients (15-49 years old) to investigate consistency of gender differentials over time and their risk factors. Of the 15,889 STD patients, the HIV prevalence ranged from 16.0% in 1990 to 41.8% in 1997. The odds ratios (ORs) of HIV infection for women compared to men decreased by age; women 15-24 years were nearly twice as likely as men of the same ages to be HIV infected (OR 1.7 [1.5-2.0]), but risk in those >44 years was almost equal (OR 0.8 [95% CI 0.7-1.2]). The odds of HIV infection for women compared to men were twice in unmarried patients (OR 2.1 [95% CI 1.8-2.3]). This association persisted after controlling for age groups or marital status, residence, level of education, and presence of STD syndromes. This pattern had been consistent over 12 years. Adolescent women with symptoms of STDs should be a focus for the HIV/STD intervention programmes because of their high risk for HIV.