This is the first report about a patient with concomitant polymyositis (PM), myasthenia gravis (MG), and aplastic anemia (AA). A 54-year-old male developed myalgia and muscle weakness, which gradually progressed over 2 months. He was persistently affected by MG and AA. Brachium magnetic resonance imaging showed increased signal intensity in the left triceps and deltoid muscles on short tau inversion recovery images. A muscle biopsy examination revealed perifascicular atrophication and inflammatory myopathy. We diagnosed the patient with PM combined with MG and AA. He was successfully treated with an autologous bone marrow transplantation (BMT). The present case suggests that BMT is a therapeutic option for PM, MG, and AA.