A 29 year-old-man who had been diagnosed as having myelodysplastic syndrome (MDS) in August 1985 was re-admitted to our hospital because of fever and palpitation. His peripheral blood showed severe pancytopenia and bone marrow findings remained to be compatible with MDS (refractory anemia), but karyotype of bone marrow cells revealed 7 monosomy in 17 of 20 metaphases examined. Other laboratory findings revealed decreased serum haptoglobin, positive urine hemosiderin and the normal resistance of red cell membrane. In addition, both Ham test and sugar water test were negative. The titer of cold agglutinin was low, Donath-Landsteiner antibody was not detected. These findings suggested the association of autoimmune hemolytic anemia (AIHA), although both direct and indirect Coombs' test were negative. After administration of 50mg of prednisolone daily, the frequency of red cell transfusion was markedly decreased and transfused red cell life span was prolonged from 10.4 days to 27 days. Afterward, his hematological status rapidly transformed into that of acute nonlymphocytic leukemia about 13 months after admission and he died of gastrointestinal bleeding and cerebral bleeding. Cases of MDS with immunological disorder have been reported. This is, however, the first case of MDS associated with Coombs negative AIHA.