The erectile responses of 13 nondysfunctional males and 48 dysfunctional males were compared during Nocturnal Penile Tumescence (NPT) and during exposure to erotic videotapes. The results showed distinct patterns of NPT and daytime responding that could differentiate the various subgroups: those displaying (i) no dysfunction; (ii) vasculogenic erectile dysfunction; (iii) high risk for erectile dysfunction (organic and psychogenic); (iv) psychogenic dysfunction; reactive to erotica; and (v) psychogenic dysfunction; nonreactive to erotica. Subjects participating in this study underwent a comprehensive medical and psychological screening to place them in each subgroup. The penile circumference response to erotic stimuli used in conjunction with NPT response appeared useful in differentiating subgroups of erectile dysfunction and suggested the need for further diagnostic refinement in this area. The majority of vasculogenic dysfunctional subjects experienced greater erection responses during exposure to erotic stimulation than during NPT; several of these subjects achieved almost full erections in waking states but were practically flaccid at night. The discussion covers diagnostic and therapeutic implications.