Dermatoscopy is used to aid in the differential diagnosis of pigmented skin lesions. The aim of this study was to identify dermatoscopic differences between atypical melanocytic naevi and thin malignant melanomas. A set of 180 difficult cases (60 thin melanomas, 120 clinically atypical benign melanocytic naevi) was analysed. Differences in structure, distribution of pigmentation, presence or absence of important structures, total number of colours and asymmetry of the lesions were identified. The three-structure type, multifocal distribution of pigmentation, eccentric peripheral hyperpigmentation, multiple colours (three or more) and asymmetry of structures/colours in the dermatoscopic image were considerably more frequent in melanomas than in benign lesions (P<0.001, chi-squared test). No single dermatoscopic criterion exists to discriminate between all melanocytic lesions with sufficient confidence. Some criteria are helpful in the differential diagnosis and management of difficult cases.