Comprehensive clinical, morphologic, and ultrastructural studies of 12 cases with various forms of carcinoma of the sweat glands permitted distinguishing the clinical nodular and papular-cystic forms. Histologically malignant analogs of benign tumors, primary adenocarcinomas, and unclassified tumors are distinguished. The histogenetic appurtenance of the tumors of the latter group may be identified only by ultrastructural studies that help detect a number of organ specificity signs. Electron microscopy used in the diagnosis of poorly differentiated tumors of the skin helps carry out adequate therapeutic strategy and permitted a better validated prediction.