Evaluate factors associated with HIV transmission risk behavior among HIV-infected adults in Uganda. Cross-sectional and nationally representative study (2004-2005 Uganda HIV/AIDS Sero-Behavioral Survey) tested 18,525 adults (15-59 years old) for HIV and herpes simplex virus type 2 (HSV-2). Data were weighted to obtain nationally representative results. Sexual-risk behavior by HIV-infected individuals was defined as unprotected sex at last encounter. Using multivariate analysis, we identified factors associated with being sexually active, knowing HIV status, and using contraception and condoms. Of 1092 HIV-infected respondents, 64% were female (median age was 33 years), 84% had HSV-2, and 13% reported one lifetime partner (1% of men and 23% of women). Twenty-one percent of adults knew their HIV status and 9% knew their partners'. Seventy-seven percent were sexually active, of whom 27% reported condom use at last sex. Of last unprotected sexual encounters, 84% were with spouses and 13% with steady partners. Of cohabitating persons, 40% had an HIV-negative spouse. Those who knew their HIV status were three times more likely to use a condom at last sex encounter [adjusted odds ratio (AOR), 3.0; 95% confidence intervals (CI), 1.9-4.7] and those who knew their partners' HIV status were 2.3 times more likely to use condoms (AOR, 2.3; 95% CI, 1.2-4.3). A minority of HIV-infected adults in Uganda knew they had HIV infection; nearly half were in an HIV-discordant relationship, and few used condoms. Knowledge of HIV status, both one's own and one's partner's, was associated with increased condom use. Interventions to support HIV-infected persons and their partners to be tested are urgently needed.