This study examined partner assessments and high-risk sexual encounters among STD clinic patients. Participants were 42 English-speaking attendees of a public STD clinic in Southern California with ages ranging between 18 and 60. Twelve focus groups were conducted. Qualitative data from the focus groups were analyzed using grounded theory. The analysis resulted in delineating five temporally-ordered stages of causal sexual encounters. Participants reported that they did not use condoms when they perceived new sexual partners as being safe. Instead of directly addressing their partner's sexual history, they relied heavily on both visual and verbal cues to judge if their partner was clean, or disease-free. This assessment process reflects an error in judgement, particularly because most of the participants had already contracted a STD. Suggestions are provided for incorporating the information about the partner assessment process into AIDS educational programmes.