Mutations in p53 were detected in 11/23 (48%) of non melanoma skin cancers in renal allograft recipients and in 5/8 (63%) of sporadic tumours from immune competent patients. 9/12 (75%) of mutations in transplant patients and all 5 mutations in non transplant tumours were consistent with damage caused by ultraviolet (u.v.) irradiation. DNA sequences, predominantly of the epidermodysplasia verruciformis (EV) subgroup, were detected in 9/23 (39%) of transplant tumours and in 2/8 (25%) of eight non-transplant tumours. There was no relationship between HPV status and p53 mutation, HPV DNA being present in 5/16 (31%) of tumours with p53 mutation and 6/15 (40%) of tumours lacking p53 mutation. These data are consistent with an important role for sunlight in the development of post-transplant skin cancer, and with limited functional data suggesting that E6 proteins of the cutaneous and EV-related papillomaviruses do not target p53 for ubiquitin-mediated degradation.