We report a 12-year-old girl with aplastic anemia accompanied by chromosome 8 trisomy during growth hormone (GH) therapy. When she was six years old she was diagnosed as idiopathic isolated GH deficiency, and GH therapy (0.175 mg/kg/week) was initiated. At the age 12, she began to exhibit petechiae in both lower limbs. Platelet count was 11,000/microL; serum hemoglobin level 11.8 gr/mL; white blood cell count 3,400/microL, with 37% neutrophils, 58% lymphocytes, 4% monocytes and 1% basophils. Bone marrow examination showed that total nucleated cell count and megakaryocyte were 17,000/microL and 0/microL, respectively, suggesting low formation. In addition, 13% of bone marrow cells contained the 3 signals of chromosome 8 marker (trisomy 8). She was diagnosed as aplastic anemia accompanied with chromosome 8 trisomy. GH therapy was stopped immediately, and simultaneous administration of methylprednisolone and anti-thymocyte globulin was initiated. Platelet count improved with treatment, and the 3-signal chromosome 8 abnormality disappeared from the bone marrow cells. The fact that a hematological adverse effect other than leukemia exists in conjunction with GH therapy warrants further investigation into possible hematological changes occurring during or after GH therapy.