Unprotected sex (UPS) among persons receiving highly active antiretroviral therapy (HAART) remains a concern because of the risk of HIV-transmission. A cross-sectional study comparing the sexual risk behaviour of 179 people living with HIV/AIDS (PLHA) receiving HAART with that of 143 PLHA receiving preventive therapy (PT) with cotrimoxazole/isoniazid was conducted in Mombasa, Kenya. Forty-five percent of all participants were sexually active in the last six months. Participants receiving PT were more likely to report > or =2 partners (13% vs.1%; P = 0.006). Participants receiving PT reported more UPS with regular partners (odds ratio [OR]: 3.9; 95% confidence interval [CI]: 1.8-8.4) and also more sexually transmitted infections (STI) symptoms (OR: 1.7; 95% CI: 1.0-2.8; P = 0.059). More than 40% of all participants did not know the HIV-status of regular partners. Therefore, HAART was not associated with increased sexual risk behaviours though considerable risk of HIV-transmission remains. HIV-care services need to emphasize partner testing and consistent condom use with all partners.