Although treatment of psoriasis with psoralen and ultraviolet A (PUVA) is associated with a long-term risk of development of cutaneous squamous cell carcinoma (SCC), the role of PUVA alone is not established, as many patients in reported series had also received treatment with other carcinogens, such as superficial X-rays or arsenic. We have recalled and examined 54 of the 63 patients still alive who have had PUVA treatment in our department, and who have been exposed to a cumulative UVA dose greater than 2000J/cm2. None of the patients had been treated with superficial X-rays or arsenicals. Ten patients (19%) had developed SCC, and 25 (46%) had histologically atypical squamous keratoses arising at body sites similar to the carcinomas. The patients with SCC were significantly older at the start of PUVA treatment than those with keratoses alone. None of the 13% of patients without PUVA lentigines had keratoses or SCCs. These results show that high-dose PUVA treatment in the U.K., even when given alone, can frequently result in the development of SCC. Further malignancies are to be expected with continued follow-up of the patients with squamous keratoses. Absence of PUVA lentigines may be a useful indicator of a lower risk of PUVA malignancy.