Patients with diabetes mellitus (DM) were more often hypogonadal than normal fasting glucose subjects. The aim of this investigation is the assessment of characteristics and psychobiological correlates of DM associated with hypogonadism (DMAH). The Structured Interview SIEDY was used along with several biochemical, psychological and instrumental investigations in a series of more than 1200 patients with erectile dysfunction (ED); 16% of whom with type II DM. Hypogonadism was defined as circulating total testosterone (T) below 10.4 nmol/l. The prevalence of hypogonadism was 24.5% in DM versus 12.6% in the rest of the sample (P < 0.0001); differences in the prevalence of hypogonadism retained significance after adjustment for age and BMI. DMAH was associated with typical hypogonadism-related symptoms, such as reduction in sexual desire, leading to a decreased number of sexual attempts, and with higher depressive symptomatology. In DMAH, testis size and LH concentrations were significantly reduced, suggesting a central origin of the disease. At penile Duplex ultrasound examination, diabetic patients and in particular hypogonadal type II diabetic patients showed lower levels of basal and dynamic (after PGE1 injection) peak systolic velocity and acceleration, when compared to the rest of the sample, even after adjustment for age and BMI. Our results show that hypogonadism is frequently associated with type II DM, at least in the 6th decade. DMAH might exacerbate sexual dysfunction by reducing libido and mood and further compromising penile vascular reactivity.