Beginning October 1985, 111 men agreed to enter a prospective study of the side effects of low dose papaverine/phentolamine therapy. A total of 46 men dropped out, 30 during the initial phase. The percentage of men with painless nodules almost consistently doubled from one followup examination to the next: 8 per cent at 1 month, 17 per cent at 3 months, 32 per cent at 6 months and 57 per cent at 12 months. The average injection frequency of those with nodules was 2 1/2 times higher than those without nodules. Of the men 41 per cent required an increased dose of medications during followup, and 40 per cent of 50 men had at least 1 abnormality of liver function, most of these involving mild to moderate elevations of alkaline phosphatase and lactic dehydrogenase. Priapism was not encountered during self-injection but it did occur twice in 329 physician-administered injections. Careful regular monitoring of patients should continue as some patients enter the second year of treatment.