Skin cancer was found in 31 of 598 patients transplanted in Oxford. No cases occurred during the first 3 years after transplantation but the prevalence rose after 12 years to 18.2 per cent. The main risk factors predisposing to skin cancer were the time after transplantation and male sex. Comparison with data from other centres suggests that exposure to ultraviolet light is a major aetiological factor in the speed of development of skin cancer. As the incidence of new cases rose progressively with time in our patients, it would seem that skin cancer is likely to become a major clinical problem as more patients enjoy prolonged survival after renal transplantation. Preventative and screening measures should be taken by transplant units both in the UK and in other countries with similar temperate climates.