This study evaluates the dysplastic hematological changes in nine patients with idiopathic thrombocytopenic purpura (ITP) in 11 attacks, before and after corticosteroid treatment. The pretreatment blood smears of patients with ITP, displayed more neutrophils with bizarre nuclei (P < 0.001), Dohle or Dohle-like inclusions (P < 0.01), irregular distribution of granules (P < 0.05), hypo-agranulation (P < 0.05), pseudo-Pelger-Huet-like cells (P < 0.01), and nuclei with chromatine clumping (P < 0.01) than the normal children. The eosinophils of ITP patients were also dysplastic, before treatment. The pretreatment diameter of the neutrophils and the percentage of macropolycytes were greater than those of the patients with viral infections and normal group (P < 0.05 for all). The percentage of neutrophils with bizarre nuclei and nuclei with chromatine clumping and the diameter of neutrophils and macropolycyte percentage increased with corticosteroid therapy (P < 0.01, < 0.01, < 0.01, and < 0.05, respectively). The neutrophil diameter, percentage of macropolycytes, and number of neutrophils with bizarre nuclei decreased within 1-4 weeks after the therapy was stopped. In the neutrophils of two patients, diploidy and hyperdiploidy were established before and on the last day of therapy, respectively, and diploidy reversed after therapy was stopped. In conclusion, ITP patients display dysplastic findings in both neutrophils and eosinophils before treatment and corticosteroids cause transient significant increase in some of the dysplastic changes in neutrophils, (C) 2000 Wiley-Liss, Inc.