In this study those features of naevi that fulfil the clinical and light microscopical criteria of dysplastic naevi have been further examined with transmission electron microscopy. The results have been compared with the structure of normal control skin and compound naevi. In dysplastic naevi most melanosomes were abnormal, with spherical melansomes, an incomplete inner structure and uneven melanin deposit, cigar-shaped melanosomes and macromelanosomes. The intraepidermal border between the nests of dysplastic naevi were uneven and the dysplastic melanocytes extended their cell bodies among surrounding keratinocytes with a tendency to invade the epidermis in an upward direction. These findings will serve as additional criteria for dysplastic naevi.