We report a 19-year-old male with acute myeloid leukemia who developed an early cytomegalovirus gastrointestinal disease with a high level of cytomegalovirus antigenemia after bone marrow transplantation. He underwent bone marrow transplantation from HLA-matched related donor. He developed acute graft-versus-host disease on day 15, and he immediately went on prednisolone. The graft-versus-host disease disappeared immediately, but he developed a severe epigastralgia on day 23. Although absolute neutrophil count was 0.5 × 109/l, the pp65 cytomegalovirus antigenemia increased up to 1120 cells/slide. The endoscope examination was performed on day 29 and it showed erosive gastritis, and microscopical examination revealed nuclear inclusion bodies and positive cells of cytomegalovirus antigen. Ganciclovir treatment was started, and it continued until negative cytomegalovirus antigenemia was confirmed. He consequently discharged to outpatient without late phase CMV diseases or recurrence of AML. We assessed that the patient had two risk factors. First, he was the cytomegalovirus seropositive patient and received bone marrow from seronegative donor. Second, he was treated with prednisolone for acute graft-versus-host disease. The ganciclovir treatment turned out to be successful, but the cytomegalovirus disease developed too early to start the antigenemia guided preemptive treatment. The high-risk patients could develop the early cytomegalovirus disease even though under the status of myelosuppression after bone marrow transplantation.