Forty-one cases of typical melanocytic skin lesions (15 intradermal nevi, 14 Spitz nevi and 12 malignant melanomas) were used to investigate the value of staining of nucleolar organizer regions (NORs) in the differential diagnosis of such pigmented lesions. Histologic sections were stained by the silver colloid (Ag) method, with and without the prior use of a melanin blocking agent. There were statistically significant differences in the mean numbers of AgNORs per nucleus between the groups of lesions studied (1.658 for intradermal nevi, 3.0042 for Spitz nevi and 6.669 for malignant melanomas). Sections treated with potassium permanganate (melanin blocking agent) prior to staining showed an obvious increase in the AgNOR scores in all groups; this increase was highest for Spitz nevi. Although AgNOR staining allows a distinction to be made between intradermal nevi and malignant melanomas, the striking overlap between the counts for Spitz nevi and malignant melanomas precludes the use of this technique as the sole method for establishing the diagnosis of malignancy. Other clinical and morphologic data are especially required to make the diagnosis of Spitz nevi.