A controlled double-blind trial was carried out on fifty-nine matched pairs of hand warts in a group of twenty-four patients, with proven resistance to treatment, to study the effectiveness of the intralesional injection of 0.1% solution of bleomycin sulphate in normal saline, compared with a normal saline placebo injected into the paired warts in the same patient. We found that 87.5% of patients showed a more favourable response to bleomycin (P less than 0.001); 76% of the fifty-nine warts treated with bleomycin were cured by one to three injections of up to 0.2 ml of solution. Subsequently 75% of thirty-two patients with resistant hand warts and 66% of fifteen patients with mosaic plantar warts were cured of all their warts. No patient received more than 4 mg of bleomycin. No toxicity was experienced. Local pain was on the whole well tolerated. This form of treatment for resistant warts is reliable, safe and acceptable to patients.