The aetiology of erectile dysfunction in men with diabetes remains unclear and is likely to be multifactorial. To explore clinical factors of possible aetiological relevance, 59 men with diabetes and erectile dysfunction (ED), referred to a sexual problem clinic, were compared with an age-matched group of non-diabetic clinic attenders with ED. Sexual interest was both higher and correlated negatively with age in the diabetic groups. There were differences in the sexual problems experienced by partners in the two groups. Both groups had received nocturnal penile tumescence (NPT) monitoring and the majority had received intracavernosal injections of PGE1 to assess capacity for erectile response. Twenty-nine percent of the diabetic men had satisfactory NPT, and most of these had other evidence of psychogenic causation. The men with diabetes were more likely to have a satisfactory response to intracavernosal injections of PGE1, and this was particularly the case among those with impaired NPTs. This difference requires explanation and may be of aetiological relevance.