A questionnaire on illness behavior was administered to a consecutive sample of 90 men and 47 women attending a public clinic for sexually transmitted diseases. Abnormal illness behavior (particularly general hypochondriasis, degree of psychologic perception of illness, and denial of stresses apart from the illness) increased with the number of previous infections. Those with no previous infections saw sexually transmitted disease as an illness significantly less often than those with previous infections. First attenders have a higher risk of noncompliance with treatment, and illness behavior may develop by association with repeated infections and the increasingly apparent connection between behavior and illness. These findings applied to homosexual as well as to heterosexual men, although responses to the questionnaire showed that homosexual men were significantly less hypochondriacal. It is concluded that there is greater evidence of psychologic maladjustment among this clinic population than among that attending general practices and that illness behavior is related to established risk factors such as more partners and previous infections.