Achieving and maintaining a penile erection are two essential components of the male sexual response. It has recently been suggested that distinct molecular mechanism could underlie the two disturbances. The aim of the present study is to verify possible clinical differences on pathogenetic factors underlying difficulties of achieving and maintaining an erection. We studied a consecutive series of 560 patients (aged 51.9+/-12.8 y old) reporting erectile dysfunction (ED), using SIEDY structured interview. Patients were classified into two distinct categories: those with difficulties in maintaining, rather than achieving, an erection (sample A) and those with main problems in achieving an erection (sample B). A complete physical examination and a series of metabolic, biochemical, hormonal, psychometric, penile vascular tests and nocturnal penile tumescence and rigidity evaluations (NPT) were also performed. Sample B patients showed a higher prevalence of organic conditions related to ED, when compared with sample A as confirmed by higher SIEDY scale 1 scores (3[1-5] vs 1[0.1-3] for sample B vs sample A, respectively; P < 0.0001) which explores organic component of ED and higher prevalence of pathological instrumental parameters. No difference among groups was observed for SIEDY scale 2 (relational component) and SIEDY scale 3 (intrapsychic component) of ED. In conclusion, this study shows for the first time that patients with difficulties in maintaining erection are less likely to be affected by organic disturbances interfering with sexual function, when compared with those unable to achieve a valid erection.