In 101 patients with non-familial cutaneous melanoma (CM), melanocytic naevi were counted and classified according to clinical criteria. Only 8% of the patients had very atypical naevi. These atypical naevi were few in number and only one patient exhibited dysplastic naevus syndrome. An histological study was undertaken on the hypothesis that, in a given individual, if the most clinically atypical naevus is not histologically dysplastic it is unlikely that any of the others are. The most clinically atypical naevus in each patient was biopsied. Estimated in this way the prevalence of dysplastic naevi in patients with non-familial CM was only 18%. Comparison of patients with and without dysplastic naevi did not suggest that they constituted two different subsets. An attempt to correlate clinical diagnosis and histological features in this group of patients showed that the diagnosis of dysplastic naevi on the basis of clinical criteria alone is difficult and not reliable.