DNA flow cytometry was performed on formalin fixed, paraffin embedded melanocytic naevi. DNA aneuploidy was detected in all three types of naevus but was significantly more frequent in those naevi accepted as precursors of malignancy: that is, dysplastic and congenital pigmented hairy naevi. It may be that the presence of DNA aneuploidy has prognostic significance in these naevi. Technical problems were encountered in the analysis of data from melanocytic lesions so that caution is recommended in interpretation of studies using formalin fixed tissue.