14 patients who developed cutaneous malignant melanoma (CMM) after renal transplantation were studied. Histological review of the primary melanomas revealed that all 10 evaluable patients had a precursor naevus from which the CMM arose, 7 of which were dysplastic, and that 13 of the 14 patients had an abnormal host response to the tumour, characterised by absence of the usual lymphocyte/ macrophage infiltrate. These observations suggest that CMM in renal-transplant recipients evolves from precursor naevi in immunosuppressed patients who are unable to mount an appropriate cellular immune response to neoplastic cells. The clinical detection of dysplastic naevi offers an opportunity to identify in advance those immunosuppressed patients who are prone to melanoma and to modify their medical management accordingly.