A 4-year-old boy with acute myeloid leukemia developed acute myositis associated with refractory thrombocytopenia one month after autologous bone marrow transplantation (BMT). Clinical, electromyographic and biohumoral features were consistent with the diagnosis of myositis. The patient responded to corticosteroids, and 39 months after BMT he is in complete remission and has regained good muscle function. Although we could not determine with certainty the specific pathophysiologic mechanism of this complication, it should be pointed out that acute myositis can occur in the early post-BMT period.