Sexually dysfunctional diabetic and nondiabetic males were compared with a group of normal controls using different endocrinological, psychophysiological, and psychological parameters. One hundred male subjects participated in this study: 47 diabetics with sexual dysfunction (DD), 31 nondiabetics with sexual dysfunction (NDD), and 22 normal controls (C). They were evaluated by an internist (physical examination and medical history), a psychologist (psychological and sexual functioning tests), a psychiatrist (psychiatric history and mental status examination), a urologist (genitourinary physical examination), and an endocrine biochemist (evaluation of endocrine factors). Additionally, subjects were evaluated for nocturnal penile tumescence (NPT) during three nights in the sleep laboratory to obtain a differential diagnosis of impotence, that is, psychogenic vs. organic. Both sexually dysfunctional groups showed significant differences on several measures in the psychological and psychophysiological evaluations. There were also significant differences between these two groups and the control group. Plasma levels of total testosterone and serum levels of prolactin, luteinizing hormone (LH) and follicle-stimulating hormone (FSH) showed no significant differences among the three groups, but there were some significant correlations between the endocrine and psychological measures. No significant correlations were found between the endocrine and psychophysiological measures.