Studies have shown that physicians' performance has not been as good as it should be in detecting sexually transmitted diseases (STDs) and in counseling patients about their transmission. The AIDS pandemic has underscored the need to find out why this is true. In our study, we identified the major reasons physicians believe other doctors fail to take adequate sex histories. Scales were then developed to measure the three principal reasons given by these physicians: embarrassment, belief that the sex history is not relevant to the patient's chief complaint, and belief by the physicians that they are not adequately trained. When 350 senior medical students were surveyed, 93% thought that knowledge of a patient's sexual practices is an important part of their patient's medical history, but 50% felt poorly trained to take this history and 25% felt embarrassed to ask the necessary questions. To learn why some students score well on these three dimensions and others do not, a limited number of personal attributes were measured and correlated with the scores on these three measures. Shyness and social anxiety as a personal trait predicted which student was most likely to experience embarrassment in taking a sex history. A nonsympathetic view of patients' psychosocial problems was the variable most closely related to the belief that the sex history was of little importance in understanding a patient's problem. Students who believed this most strongly were the same ones who were most homophobic, authoritarian, and had the greatest fear of AIDS infection. The sense of not feeling adequately trained to take a sex history related most strongly to low self-esteem. How these barriers to STD risk assessment might be overcome is discussed.