To determine the truthfulness of patients' and research subjects' self-reports of their sexual and drug use histories in studies of human immunodeficiency virus (HIV) transmission and acquired immunodeficiency syndrome (AIDS) risk factors, studies of or pertinent to lying about AIDS risk factors were extracted from MEDLINE and PSYCLIT. The present paper describes normal and pathological motives for misrepresenting risk factors, and reviews the literature on such underreporting. There is much evidence for lying about anal intercourse and intravenous drug use risk factors, implying that the estimates of risk for vaginal transmission of HIV (particularly in Pattern-I industrialized countries) have been inflated. Research on HIV/AIDS behavioural risk factors must include sophisticated methods for the assessment of self-report validity, such as the use of behavioural markers, improved lie scales and interview methods, and physical methods.