The distribution of four main arterial risk factors (ARF) (diabetes, smoking, hyperlipidaemia (HLP), and hypertension) was investigated in 440 impotent men (mean age 46.8) in whom the penile blood-pressure index (PBPI) (ie, the ratio of the lowest systolic pressure in one of the four main arteries of the penis to the systolic pressure in the arm) was measured. In 222 the cause (organic or functional) of impotence was sought by further investigations, such as cavernosonography. 80% of this subgroup had organic impairment of erection. In 53% of these there was evidence of an arterial lesion. Smoking (64%), diabetes (30%), and HLP (34%) were all significantly more common in the 440 impotent men than in the general male population of a similar age. Whenever two or more ARFs were present mean PBPI was significantly lower. The frequency of organic impotence increased from 49% in the absence of any ARF to 100% in patients with 3 or 4 ARFs. It is concluded that increase in the frequency of impotence with age is mainly related to arteriosclerotic changes for the arteries of the penis and that the ARF and PBPI should be evaluated first in any patient complaining of impotence.