De novo AML with trilineage myelodysplasia (AML/TMDS) is reported to account for 10-15% of de novo AML and respond poorly to conventional intensive chemotherapy, In our series, 12 (25%) of 48 patients with de novo AML were diagnosed as AML/TMDS. We found that the platelet count was significantly higher (p < 0.05), and the blast percentage of the bone marrow was significantly lower (p < 0.05) in the AML/TMDS group than in the AML/non-TMDS group. Sex ratio, age, WBC and RBC count did not significantly differ between the two groups. The immunological markers and the myeloperoxidase positivity of the blasts of AML/TMDS varied widely. The CR rate was 66.7% in the AML/TMDS group and 83.3% in the AML/non-TMDS group. Dysplastic changes were still detected in the bone marrow smears in 7 of 8 AML/TMDS cases who achieved complete remission. The AML/TMDS group showed significantly shorter CR duration (median; 169 days) and survival (median; 511 days, p < 0.05). However, in two cases which underwent allogeneic bone marrow transplantation (allo-BMT) during early relapse phase the disease-free survival has extended over 4 years and 2 years 8 months, respectively. Thus, we would like to propose that allo-BMT should be performed as early as possible to overcome poor outcome of AML/TMDS.