Papillomaviruses are strongly implicated in squamous cell carcinomas arising on mucosal surfaces of normal individuals, and in the skin carcinomas of epidermodysplasia verruciformis suffers. Renal transplant recipients often have numerous skin warts and, in Australia particularly, a very high risk of developing cutaneous squamous cell carcinoma. To determine the magnitude of this risk, and to test whether papillomaviruses are specifically associated with these cancers, we examined 188 renal transplant recipients for skin cancers and tested 235 biopsy specimens of (histologically proven) squamous cell carcinomas for the presence of viral DNA. The risk of developing squamous cell carcinoma increased with duration of transplant: the probability being 25% after 9.5 years (standard error = 1.3 years) rising to 50% at 20.6 years (standard error 6.8 years). Factors which did not appear to affect the risk of tumour development included the patients sex and their skin type. However the age at transplant significantly altered the risk with patients transplanted at greater than 35 years developing tumours about four times more rapidly than patients less than or equal to 35 years. Extensive hybridisation tests for the presence of papillomavirus DNA in squamous cell carcinomas were negative, as was the polymerase chain reaction amplification method using general L1 gene oligonucleotide primers. Our data do not support a role for papillomavirus in the maintenance of cutaneous squamous cell carcinoma.