In 28 patients, nonmelanoma skin cancers developed in areas previously exposed to grenz rays. In 17 patients who did not have psoriasis, no other relevant carcinogenic exposure could be incriminated. Women were more often affected than men. Most of the tumors were basal cell cancers, and most of the patients had multiple tumors. No threshold dose could be established. The distribution of the latency time among patients without psoriasis was strictly normal (median 18 years). These observations suggest that usual therapeutic doses of grenz rays, as a single agent, are capable of causing skin cancer, but only in those persons who are abnormally sensitive to x-rays.