A series of 233 consecutive primary cutaneous melanomas was histologically and clinically studied. Histologically, 53 melanomas (22.7%) were associated with naevus cells. Such a high degree of association suggests that melanocytic naevus may be a precursor of a large number of melanomas. Analysing the cases according to Clark's levels and Breslow's index, a decrease in the naevus-melanoma association was seen with tumour progression, suggesting that advanced tumours may overgrow pre-existing nevus cells, appearing as de novo melanomas. The comparison between histological and clinical data suggest some interpretations of the natural history of melanoma.