To confirm reports that skin cancer can be prevented with retinoids, we conducted a three-year controlled prospective study of oral isotretinoin (also called 13-cis retinoic acid) in five patients with xeroderma pigmentosum who had a history of multiple cutaneous basal-cell or squamous-cell carcinomas. Patients were treated with isotretinoin at a dosage of 2 mg per kilogram of body weight per day for two years and then followed for an additional year, without the drug. Before, during, and after treatment, biopsies of all suspicious lesions were performed, and skin cancers were surgically removed. The patients had a total of 121 tumors (mean, 24; range, 8 to 43) in the two-year interval before treatment. During two years of treatment with isotretinoin, there were 25 tumors (mean, 5; range, 3 to 9), with an average reduction in skin cancers of 63 percent (P = 0.019). After the drug was discontinued, the tumor frequency increased a mean of 8.5-fold (range, 2- to 19-fold) over the frequency during treatment (P = 0.007). Although all patients experienced mucocutaneous toxic effects, and triglyceride, liver-function, or skeletal abnormalities developed in some, high-dose oral isotretinoin was effective in the chemoprophylaxis of skin cancers in patients with xeroderma pigmentosum.