Electron microscopy (EM) and electron microscopic cytochemistry have shown that diffuse melanosis in advanced metastatic melanoma is the result of an unlimited spread of single melanoma cells throughout the dermis; these cells retain their melanogenic capacity and continue to produce melanized melanosomes which eventually are deposited within dermal macrophages and thus impart to the skin a diffuse slate-blue color. These findings are in accordance with previous observations in a similar patient and thus support the concept that single cell metastasis represents a common pathogenic event in these patients.