An unusual case of melanosis associated with metastatic malignant melanoma is reported. This was characterized by progressive blue/gray discoloration of the skin of the chest and abdomen in an elderly patient, 1 year after removal of a polypoid malignant melanoma from the right arm. A biopsy of involved skin revealed perivascular aggregates of melanin-laden histiocytes throughout the dermis, the histopathologic hallmark of melanosis. An unusual aspect of the case was the coincidental finding of a tumor embolus within a small dermal vessel, probably a lymphatic. To date, neoplastic melanocytes have been detected in only a small minority of skin biopsies with features of melanosis. This case and a distillation of related information in the literature lead to the conclusion that the essence of melanosis, and the feature that distinguishes this from conventional metastatic melanoma, is the persistent and cumulative dissemination of melanin, via the bloodstream, throughout the body. This in turn leads to progressive pigmentation of all internal organs and the skin. Only continuous access to the circulation by neoplastic melanocytes could explain such a phenomenon. Potential mechanisms by which this could arise are discussed in the context of existing knowledge.