In Kenya in 1999, an estimated 6.9% of women nationally said they had exchanged sex for money, gifts or favours in the previous year. In 2000 and 2001, in collaboration with sex workers who had formed a network of self-help groups, we conducted an exploratory survey among 475 sex workers in four rural towns and three Nairobi townships, regarding where they worked, the number of clients they had and the risks they were exposed to. Participants were identified by a network of social contacts in the seven centres. Most of the women (88%) worked from bars, hotels, bus stages and discos; 57% lived with a stable partner and almost 90% had dependent children. In the previous month, 17% had been assaulted and 35% raped by clients. Unwanted pregnancy was common; 86% had had at least one abortion. Compared with women in rural towns, township sex workers were younger (median age 22 vs. 26), saw more clients (median 9 vs. 4 per week) and earned more from sex work (up to 63-90 euros vs. 12 euros per week). Issues of alternative sources of income, safety for sex workers and the conditions which create the necessity for sex work are vital to address. The question of number of clients and the nature of sex work have obvious implications for HIV/STI prevention policy.