Primary adenocarcinoma of sweat glands is a rare tumor; approximately 220 cases have been reported in the last 30 years. We reviewed the charts of patients with primary diagnosis of this tumor treated at the Mayo Clinic between 1935 and 1995. We included only cases with initial histology slides available for re-examination. Tumors were classified into five recognizable histologic patterns (solid, ductal, mucinous, microcystic adnexal, and adenocystic carcinoma) and graded by the Broder system. Statistical analysis consisted of Kaplan-Meier product limit method and Cox multiple regression test. In total, 55 patients were identified, and age ranged from 13 to 85 years (mean 59 years). Thirty-six patients (65 percent) presented to the Mayo Clinic for initial treatment; all except one had disease limited to the primary site. Microcystic adnexal carcinoma was the most frequent type, and more than 50 percent were grade 2 tumors. Among these 36 patients, 4 had some type of recurrence. Patients who developed metastasis had a high-grade tumor in the initial biopsy. Nineteen patients were referred with recurrence; 13 had local recurrence, 4 had regional diseases, and 2 had distant metastases. The histologic distribution showed 47 percent solid tumors, and 37 percent of them were grade 3. Multiple regression analysis did not show a difference in recurrence or survival when gender, age, tumor location, or histologic pattern was evaluated. In addition, there was no difference in the outcome between wide surgical resection and micrographic surgery. The only predictive factor for distant metastases and/or death (p < 0.003) was histologic grade. Overall 10-year survival rate was 86 and 60 percent for primary and referred patients, respectively. We conclude that histologic diagnosis of sweat gland carcinoma must be complemented by clinical examination to evaluate metastases. Clinical behavior depends on the histologic type of tumor, degree of differentiation, and clinical stage. On recurrence, the likelihood of further recurrences and mortality increases dramatically. Aggressive initial local ablation with tumor-free margins is recommended. In high-grade tumors, prophylactic regional lymph node dissection may further characterize tumor aggressiveness and may justify adjuvant radiotherapy as part of the primary treatment.