Of 22,274 patients > or = 12 years old attending a Nairobi primary health care (PHC) clinic, 1076 (4.8%) had STD-related complaints, of whom 980 underwent assessment of risk factors for human immunodeficiency virus (HIV) infection and infrequent condom use. Gonorrhoea, chancroid, syphilis seroactivity, trichomoniasis, or objective signs of STD were found in 78%, and HIV seropositivity in 15% of men and 19% of women. Most women were married, living with a spouse; while most men were single, or married, but living separated from a spouse. Among married men, last sex was with a female sex worker (FSW) or casual partner for 60% not living with a spouse and 26% living with a spouse (P<0.005). Two or more partners during the past year were reported by 82% of men and 25% of women (P <0.001), and 55% of men and 11% of women reported the last partner was high risk. HIV seropositivity among both genders was associated with numbers of partners, and among women, with being widowed or divorced. Only 3% reported use of a condom with the last partner. Among men whose last sex was with a FSW, 74% said the reason for not using a condom was not having one. Thus, infrequent condom use, low condom availability, and gender differences in behaviour necessitate modifying development policies that separate families; and better coordination between family planning, PHC, and AIDS/STD programmes, with improved supply, social marketing and community-based distribution of condoms in high-risk settings for STD/HIV prevention.