A total of 761 melanocytic lesions were studied to elucidate the usefulness of clinical features for the diagnosis of dysplastic naevi. Characteristics associated with high (irregular border, irregular pigmentation), intermediate (black coloured areas, largest diameter > 0.5 cm, change of size, change of colour) and low diagnostic efficiency could be defined. Combinations of criteria had high sensitivities: at least one of the following four criteria was positive in 96% of the dysplastic naevi and in all melanomas with less pronounced clinical characteristics: irregular border, irregular pigmentation, greatest diameter >0.5 cm, black coloured areas. A lesion is therefore unlikely to be a dysplastic naevus or a melanoma if all these criteria are absent. When change of size and change of colour were analysed in addition to the features mentioned above a sensitivity of 0.96 was found for at least two of these six criteria. At least three of these six criteria were observed in all melanomas with less pronounced clinical characteristics. However, a rather low specificity (0.19 for at least one of four positive criteria, 0.20 for at least two of six positive criteria) indicated that dysplastic and non-dysplastic naevi cannot be clinically differentiated with acceptable certainty. With less stringent histological criteria approximately twice as high specificities were found. Specificities were about twice as high in a subgroup of patients with at least one proven dysplastic naevus besides the lesion under diagnostic consideration. This facilitates the identification of individuals at risk of developing a melanoma.