We estimate changes in sexual behavior for HIV-positive individuals enrolled in an AIDS treatment program using longitudinal household survey data collected in western Kenya. We find that sexual activity is lowest at the time that treatment is initiated and increases significantly in the subsequent six months, consistent with the health improvements that result from ART treatment. More importantly, we find large and significant increases of 10 to 30 percentage points in the reported use of condoms during last sexual intercourse. The increases in condom use appear to be driven primarily by a program effect, applying to all HIV clinic patients regardless of treatment status.