Renal allograft recipients have an increased incidence of malignancy including squamous carcinoma of cervix and skin. There is growing evidence that human papillomavirus (HPV) has a part to play in malignant transformation at these sites. We have previously identified HPV DNA in the skin and genital lesions of such patients by dot and Southern blotting. In situ hybridization studies, using biotinylated DNA probes for HPV 4, 5 and 8 in skin lesions and 6, 11, 16 and 18 in genital lesions, were performed on tissues derived from the same group of patients. In the cutaneous lesions, only 25% of the specimens probed were found to contain virus by in situ hybridization; 60% of these specimens were found to harbour virus by dot and Southern blotting. In situ hybridization revealed HPV 16 and/or 18 in 86% of the genital lesions probed.