Apoptosis of peripheral leukocytes in 9 patients with myelodysplastic syndromes (MDS) was examined in vitro, using peripheral blood that had been gently incubated at 37 degrees C for 5 hours. The MDS patients included 3 with refractory anemia, 2 with refractory anemia with ringed sideroblasts, and 4 with refractory anemia with excess blasts. Peripheral blood specimens were also obtained from a control group consisting of 10 patients with iron deficiency anemia (IDA), 10 with idiopathic thrombocytopenic purpura (ITP) and 10 healthy individuals. Apoptotic granulocytes (Apo-Gs) were identified by morphological changes, including nuclear fragmentation, and expressed as a percentage of every 300 granulocytes counted. Apo-Gs were counted 1, 2, 3, 4, and 5 hours after incubation. Although the percentage of apo-Gs climbed over time in the MDS patients, a small number of apo-Gs were also observed in the healthy individuals. In the MDS patients, the proportions of apo-Gs 5 hours after incubation (37 degrees C) were significantly higher than those in the IDA and ITP patients and healthy individuals (15.7 +/- 8.0% in MDS patients vs. 2.8 +/- 1.2% in IDA patients, 2.3 +/- 1.7% in ITP patients, and 0.7 +/- 0.6% in healthy individuals; p < 0.005). No significant differences were observed in the proportions of apo-lymphocytes. DNA fragments were observed in blood lymphocyte from an MDS patient examined. Negative correlations between the percentages of granulocytes and Apo-Gs tended to be observed in the MDS patients. These results suggest that a strong susceptibility to peripheral granulocyte apoptosis is one of possible causes of granulocytopenia in MDS patients.