Common and uncommon variants of melanocytic naevi.

Numerous variants of melanocytic naevi have been described. Their main pathological significance lies in their distinction from melanoma, as well as being precursors and risk markers for melanoma. Various degrees of atypia such as cytological atypia, architectural disorder and pagetoid spread (pagetoid melanocytosis) may be present in naevi and need to be recognised as appropriate for the subtype. As well as the distinction from melanoma, naevi must be differentiated from atypical lesions, such as atypical Spitz tumours, which do not fulfil all the criteria of melanoma, may be benign or malignant and have been called 'melanocytic tumours of unknown malignant potential'. The diagnostic grey area also includes a group of benign atypical naevi which are difficult to subclassify into specific entities. In this paper naevi are divided into: firstly, the common acquired group with a brief discussion of junctional, compound and intradermal naevi, minor variants such as halo and balloon naevi and the major variants that may cause problems -- dysplastic naevi, naevi of special sites, recurrent naevi and Spitz naevi; secondly, congenital naevi; thirdly, blue naevi and related lesions (dermal melanocytoses); and finally, combined naevi. The emphasis is on diagnostic pathological features and the differential diagnosis with melanoma.