Seventeen cases of pure red cell aplasia (PRCA) with thymoma were studied clinically, histologically, and immunologically. Two cases were associated with myasthenia gravis (MG), and three with hypogammaglobulinemia. Coombs test and antinucleus antibody test were positive in five cases. All thymomas were spindle cell types, and the adjacent thymuses had no germinal center, but showed epithelial clusters frequently. All patients, except one whose tumor was unresectable, had thymo-thymomectomy. The operation was effective in six cases (37.5%), and the effects were not different between two operative procedures (simple and extended thymectomy). Myasthenic symptoms in two patients remitted after the operation, but effects on hypogammaglobulinemia were conincident with those on PRCA.