We treated 23 patients with myelodysplastic syndromes (MDS); 2 refractory anemia (RA) with prior therapy, 11 RA with excess of blasts (RAEB), and 10 RAEB in transformation (RAEB-T), with daily oral 45 mg/m2 all-trans retinoic acid (ATRA) in a multiinstitutional prospective study. In two patients with RAEB and one with RAEB-T, a more than 1,000/microL increase of peripheral neutrophil counts was observed with some reduction of blast percentage in the bone marrow 2 to 9 weeks after the start of ATRA. However, the effect was transient and did not last for more than 5 weeks despite the continuation of ATRA therapy. In one other patient with RA, one patient with RAEB, and one patient with RAEB-T, slight increase of hemoglobin levels or reduction of blast percentage in bone marrow was noted. Toxicities attributable to ATRA were minimal and included cheilitis, xerosis, dermatitis, gastrointestinal disorders, abnormal liver function tests, and high serum triglyceridemia. Although ATRA works remarkably as a differentiation therapy in acute promyelocytic leukemia, its effect in MDS included in this study was modest. Further study of this agent alone or in combination may be warranted in less advanced stages of this disease.