Eighteen patients with myelodysplastic syndrome received 13-cis-retinoic acid (1.0-mg/kg/day starting dose with 0.5-mg/kg increment escalations) in a phase I-II trial. Two partial responses involving the erythroid series were observed in four patients with primary refractory anemia with ring sideroblasts. One of two patients with chronic myelomonocytic leukemia also achieved a partial response. No other responses were found in the remaining patients, which included eight with refractory anemia with excess blasts. In six patients drug toxicity necessitated termination of the trial. Four patients had unexpected drug-induced thrombocytopenia; three of these had low platelet counts before treatment. Two of the six patients had other toxic effects. Further studies are warranted to evaluate the effectiveness of 13-cis-retinoic acid in patients with refractory anemia with ring sideroblasts and chronic myelomonocytic leukemia. At moderate doses significant toxic effects, including thrombocytopenia, are not uncommon.