The relationship between common acquired naevi and malignant melanoma of the skin was assessed in a case-control study. In 183 patients and 183 controls matched for sex, age and area of residence, there was a strong association between the presence of pigmented naevi on the arms and melanoma. In comparison with persons having no naevi on the arms, the crude relative risk of melanoma was 28.0 when naevi were present. After adjusting for hair colour, propensity to sunburn and lifetime sun exposure, the final risk estimate was 30. 1. Family history did not appear to be a determinant of disease independent of the above risk factors.