We studied the psychological and biological correlates of hypoactive sexual desire (HSD) in a consecutive series of 428 patients with erectile dysfunction (ED), by using the structured interview SIEDY. A complete physical examination and a series of biochemical, hormonal, psychometric, and penile vascular tests were also performed. Among the patients studied, 22.8% reported a mild, 12.9% a moderate, and 4.6% a complete loss of sexual interest. Patients reporting HSD showed significantly lower testosterone (T) levels than the rest of the sample, although the prevalence of hypogonadism (T<10 nM) was comparable in the two groups. Only a minority (<2%) had severe hyperprolactinemia (>700 mU/l), which, nonetheless, was closely associated with a relevant HSD. Both mental disorders and use of medication interfering with sexual function were significantly associated with HSD, as well as depressive and anxiety symptoms. HSD patients showed significantly higher scores in SIEDY scale 2, which explores the relational component of ED. In particular, perceived partner's libido and climax were crucially associated with an impairment of patients' sexual desire. In conclusion, HSD in ED is associated with several biological, psychological, and relational factors that can be simultaneously identified and quantified using the SIEDY structured interview.