Two patients with multiple basal cell carcinomas, due either to the nevoid basal cell carcinoma syndrome (NBCCS) or arsenical insecticide exposure, were treated with oral isotretinoin for 7 or 8 years, respectively. Gradually decreasing dosage levels were employed. During the initial courses of therapy, high doses (2.0–3.0 mg/kg/day) were intended as chemotherapy. In these patients only 6 of 40 (15%) lesions underwent complete clinical regression. In subsequent courses aimed at chemoprevention, the dose was progressively reduced from 1.5 to 0.25 mg/kg/day. During therapy, no new lesions were observed in the patient with the arsenical exposure. The NBCCS patient developed 1 new lesion during therapy at 1.0 mg/kg/day, 1 new lesion at 0.5 mg/kg/day and 5 new lesions at 0.25 mg/kg/day. Treatment was discontinued and the patient with the arsenic exposure developed his first new tumor 17 months afterwards; in contrast, the NBCCS patient developed 29 tumors within 13 months. These findings suggest that long-term therapy with isotretinoin is needed for the continuation of the cancer chemopreventive effect. However, the need for continuous rather than intermittent maintenance therapy, and the determination of the optimal dose for this purpose may depend on the etiology of the multiple carcinomas and on the tolerability of the lowest effective dose by the individual patient. With these encouraging data, it now appears appropriate to expand this pilot study and perform larger trials to dertermine the usefulness of isotretinoin in the chemoprevention of basal cell carcinoma in patients with multiple tumors.