Dysplastic features of peripheral blood granulocytes were studied to investigate the diagnostic value estimating cytoplasmatic hypogranulation and nuclear abnormalities of the pelgeroid type in myelodysplastic syndromes (MDS). Hypogranulation was measure both as the percentage of agranular neutrophils and as a score value, taking into account also cells with slight or moderate hypogranulation. We studied 62 cases of MDS (18 refractory anemia, 11 sideroblastic anemia, 26 refractory anemia with excess of blasts, three chronic myelomonocytic leukemias, four refractory anemia with excess of blasts in transformation). For comparison we studied 13 cases of myeloproliferative disorders, 18 patients with different forms of anemia and 20 normal controls. Reference values were defined as the 95% probability limit of the mean values of normal controls. In MDS 52/62 patients (84%) had increased numbers of pelgeroid cells, 40/62 (65%) had abnormal granulation scores while only 14/62 (23%) had increased percentages of agranular neutrophils. The mean granulation score (+/- S.D.) in MDS (225.0 +/- 57.4), was significantly lower (p less than 0.001) than in myeloproliferative disorders (282.7 +/- 9.0), anemias (288.8 +/- 8.0) and normal controls (281.7 +/- 12.9). Pelgeroid neutrophils were significantly (p less than 0.001) more common in MDS (11.6% +/- 7.8) compared to myeloproliferative disorders (1.1% +/- 1.0), anemias (3.1% +/- 2.0) and normal controls (1.9% +/- 1.5). There was no significant correlation between the degree of hypogranulation and the percentages of pelgeroid cells in individual patients. Hypogranulation tended to be more pronounced in the more immature forms of MDS while pelgeroid cells were equally common in the different subgroups. When the two parameters were combined peripheral blood dysplasia was recognized in 92% of the MDS cases. The results suggest that quantitative estimation of hypogranulation and of nuclear abnormalities in peripheral blood polymorphs are simple and valuable diagnostic tools in MDS.