Forty patients with myasthenia gravis underwent maximal thymectomy. Complete histologic study findings were available for 38 patients. The prevalence of ectopic thymic tissue was 39.5% (15 of 38). On the basis of the presence or absence of ectopic thymic tissue, patients were divided into two groups: group I had ectopic thymic tissue and group II had no ectopic thymic tissue. Male/female ratio was almost equal (1.1:1) in group I, whereas in group II the ratio was 1:2.8. The duration of the disease was less than 1 year in 80% of group I and 47.8% of group II patients (p = 0.05). Furthermore, ectopic thymic tissue (group I) was associated with poor outcome of operation (p = 0.003). Only 2 (13.3%) of 15 patients in group I had complete remission as compared with 11 (47.8%) of 23 patients in group II. Thus it appears that the presence of ectopic thymic tissue not only modifies some of the clinical parameters of myasthenia gravis, but also could serve as a prognostic factor in predicting the outcome of operation.