Pure red cell aplasia is manifested by unregenerative anemia with intense erythroid hypoplasia in the bone marrow and absolute reticulocytopenia. Its association with chronic myeloproliferative syndrome (CMS) is infrequent. The appearance of both processes may or may not be simultaneous and accidental association cannot be ruled out. Two cases of pure red cell aplasia were studied; one associated to Philadelphia positive chronic chromosome myeloid leukemia and the other associated to atypical CMS. In the first case no response was obtained with prednisone alone or associated to cyclophosphamide. In the second patient treatment with danazol was not efficient, however, corticotherapy produced a positive and maintained response. Treatment of pure red cell aplasia associated to CMS must be the same as that employed when this disease is presented as isolated or associated to other processes.