We assessed why heterosexually active adults did not have "safer sex" with their last sexual partner. Subjects enrolled in HIV education and testing trials at a sexually transmitted disease (STD) clinic and a university student health service (SHS) completed questionnaires about their last sexual partner's risk factors for HIV and whether they had safer sex with this partner. Of the 652 sexually active subjects, 61% reported not having safer sex with their last sexual partner and explained why. Low perceived risk of HIV infection was the most common reason, indicated by 62%, though most knew too little about their partner to ensure the encounter was low risk. Other reasons included condom unavailability (20%), the subject "didn't want to" use a condom (19%), "couldn't stop ourselves" (15%), the partner's influence (14%), and alcohol or drug use (11%). Thirty-one percent of subjects indicated more than one reason for not having safer sex. SHS subjects more often reported that the encounter was low risk for HIV transmission (p = 0.0001), while STD subjects more often reported condom unavailability (p = 0.002) and drug and alcohol use (p = 0.003). We conclude that there are many different factors promoting sexual behavior at risk of infection, combinations of which are important, and that these factors differ between samples. Preventive interventions must focus on the factors most important to the targeted population and may need to consider multiple factors simultaneously.