Regression in epithelial skin tumours is a common phenomenon, and this may be partial or complete. In keratoacanthoma and familial self-healing epithelioma, nearly all the tumours regress completely. The incidence of total regression in melanoma and basal cell carcinoma is unknown, but in 25% of melanomas and 50% of basal cell carcinomas there is evidence of partial regression on histological examination. Previous studies carried out in our laboratories have indicated that regression of skin tumours is likely to be mediated by activated CD4+ T lymphocytes, possibly via cytokine secretion.