GCMN may be precursors of melanoma, and it has been suggested that the presence of atypical foci could increase the risk of malignant transformation. In order to better define atypical GCMN, we analyzed DNA content and proliferative activity in 21 samples of GCMN, with (n=13) or without (n=8) cytologic atypia. Six benign acquired naevi (AN) and 6 malignant melanoma (MM) were used as controls. DNA content was determined with the CAS 200 image analyzer, and DNA histograms were classified according to the Auer classification. The proliferative indices (PI) were measured after Ki 67 immunostaining using the CAS 200 system. All AN and GCMN without atypia showed class I histograms (normal DNA content) and low PI (mean 1.9 and 2.1). Atypical GCMN showed in 10 cases an abnormal DNA content (class III or IV histograms) with low PI (mean 2.7), and in 3 cases a normal DNA content (class I histograms) with higher PI (mean 16.2). All MM displayed abnormal DNA content and high PI (mean 32.6). In conclusion, abnormal DNA content seems to correlate with cytologic atypia in GCMN. Atypical GCMN exhibit an overall pattern of DNA content and cell proliferation intermediate between non-atypical naevi and MM.