Merkel cell carcinoma was diagnosed in 10 patients (eight females and two males) with a mean age of 66 years and a range of 44-84. The most common sites of the primary tumor were the skin of the limbs, girdles, and head. Sixty percent of the cases had recurrences and 50% regional nodal metastases that appeared within 1 year. Of the patients treated variably with surgery, chemotherapy, and radiotherapy and followed for a mean period of 28 months, two were alive and free of disease, two were alive with disease after 3 and 6 years, and two have died of metastatic disease to the lungs and liver. The primary lesions showed quite distinctive clinical and gross features in terms of size, shape, and color. Histologically, the growth was usually diffuse and occasionally trabecular or pseudoglandular; infiltration was predominantly in the papillary and reticular dermis. The uniform, poorly cohesive cells contained argyrophilic granules in the cytoplasm and round to oval nuclei with frequently indented membranes. The mitotic index was high. Ultrastructurally, in two cases, two cell types were identified that probably represented the neoplastic counterpart of normal Merkel cells in different stages of maturation. Both cell types contained cytoplasmic neurosecretory granules and intermediate filaments, even though their cytoplasms were differently shaped.