Fifty-seven examples of a rare eccrine sweat gland tumor (aggressive digital papillary adenoma and adenocarcinoma) were studied by means of light microscopy, electron microscopy, and immunoperoxidase techniques. The neoplasm occurred as a single, painless mass, almost exclusively on the fingers, toes, and adjacent skin of the palms and soles. Microscopic features were distinct from those of other eccrine sweat gland tumors and often led to the diagnosis of such metastatic carcinoma as that of the breast. The characteristic histologic features included tubuloalveolar and ductal structures with areas of papillary projections protruding into cystic lumina. The stroma varied from thin, fibrous septae to areas of dense, hyalinized collagen. Forty tumors were classified as adenoma (ADPA) and 17 as adenocarcinoma (ADPAca). Histologically, ADPAca was distinguished from ADPA by its poor glandular differentiation and by necrosis, cellular atypia and pleomorphism, invasion of soft tissue and bone, and invasion of blood vessels. Eighteen (50%) patients with ADPA and 8 (47.0%) who had ADPAca developed recurrent lesions (2 months to 9 years) after surgical removal of the tumor. Seven (41.2%) patients with ADPAca developed metastases, of which 5 involved the lung. Three patients died of metastases, 5 to 20 years after surgical treatment of the primary tumor. The histologic malignant features in ADPAca are indicative of potential for distant metastasis and fatality. The recognition of aggressive digital papillary adenoma and adenocarcinoma as a distinct clinicopathological eccrine sweat gland neoplasm is important because of the potential for aggressive local growth and distant metastasis.