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Objectives: To determine whether malignant thymoma is associated with high rates of concomitantly occurring autoimmune diseases. Methods: Sheba Medical Center computer records from 1966 to 1995 werereviewed to identify patients with malignant thymoma, either type I (invasive thymoma) or type II (thymic carcinoma). All patients who had malignant thymoma and autoimmune phenomena were analyzed. The diagnosis of thymic neoplasm was confirmed by two independent pathologists. The diagnosis of autoimmune diseases was based on both clinical and serological findings. Results: Six of 22 (27%) cases of malignant thymoma had an autoimmunedisease. Five patients had type I malignant thymoma and either myasthenia gravis (four patients) or Graves' disease (one patient). Only one patient had type II malignant thymoma with Sjbgren's syndrome. The diagnosis of autoimmune disease preceded the diagnosis of thymic neoplasm in four cases, and was diagnosed simultaneously in two. Conclusions: Malignant thymomas are highly associated with autoimmunediseases, as are benign thymomas. To our knowledge, we report the first documented cases of a patient with thymic carcinoma and Sjogren's syndrome, and another with invasive thymoma and Graves' disease.