Pure red cell aplasia (PRCA) is well known to be concomitant with myasthenia gravis (MG), but it is difficult to predict the development of PRCA in patients with MG. Of 135 patients with MG, four (2.9%) had PRCA. All patients developed PRCA after thymectomy during a period when MG was in remission. The frequencies of thymoma, bulbar involvement and high anti-acetylcholine receptor (AChR) antibody levels were significantly higher in four patients with PRCA. Japanese MG patients with these characteristics at presentation were at significant risk for the development of PRCA after thymectomy.