Spontaneous regression in Merkel cell (neuroendocrine) carcinoma of the skin. In two Japanese women, 68 and 88 years old, Merkel cell (neuroendocrine) carcinoma of the face developed. Their tumors regressed after biopsy was performed, a rare occurrence. Histological and electron microscopic examination showed apoptosis, cellular necrosis, and an infiltration composed mainly of lymphocytes in the tumors. These changes may have been related to the mechanism of regression. It is interesting that our two patients were women, as was another patient described with Merkel cell carcinoma regression, in light of the fact that the prognosis of this tumor is sex dependent. 