We report on 22 patients with myelodysplastic syndrome (MDS), all of whom showed striking marrow fibrosis. Variable blood counts, often with teardrop poikilocytosis and a leukoerythroblastic picture, were present at diagnosis. Visceral enlargement was detected in 17 patients with a distinct splenomegaly in seven cases. All cases demonstrated dysplasia in at least two cell lineages. No specific cytogenetic abnormality seems to characterize this group of patients. Southern blot analysis showed no breakpoint cluster region rearrangement as observed in classical chronic myeloid leukemia. Ferrokinetic studies revealed quantitatively deficient erythropoiesis in all except two cases and an abnormally high fraction of ineffective erythropoiesis in all. Splenic erythropoiesis was present in eight patients. The median survival was 18 months. At the time of this report, 12 patients had died. The causes of death were disease progression (7 patients) and infection (5 patients). One might speculate that the present series of cases represents a transition between MDS and myeloproliferative disease, thereby displaying characteristics of both groups of diseases.