We investigated the impact of a short-lived policy of charging fees to patients attending public-sector outpatient health facilities in Kenya by collecting data on attendance at Nairobi's Special Treatment Clinic for sexually transmitted diseases (STDs) before (23 months), during (9 months), and after (15 months) the user-charge period. During the user-charge period, the seasonally adjusted total mean monthly attendance of men decreased significantly to 40% (95% CI 36-45) of that before fees were levied. Attendance rose in the post-user-charge period, but reached only 64% (59-68) of the pre-user-charge level. For women, the adjusted total mean monthly attendance during the user-charge period was reduced significantly to 65% (55-77) of the pre-user-charge level. Mean monthly attendance by women rose in the post-user-charge period to 22% (9-37) above the pre-user-charge level. There was no evidence of an increase in attendance over the course of the user-charge period among either men or women. The introduction of user fees probably increased the number of untreated STDs in the population, with potentially serious long-term health implications. The user-fee experience in Kenya should be carefully evaluated before similar measures are introduced elsewhere.