We conducted a phase 1-2 clinical trial to evaluate the effect of pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF) in patients with chronic idiopathic thrombocytopenic purpura (ITP) refractory to standard therapy who had platelet counts below 30 x 10(9)/L. Four patients received PEG-rHuMGDF (0.5 microg/kg of body weight per day) by daily intravenous injection for up to 7 days. Administration of PEG-rHuMGDF increased platelet counts in 3 patients. A striking thrombocytosis occurred in 2 patients, whose platelet counts were elevated to more than 700 x 10(9)/L a week after the last administration of PEG-rHuMGDF and returned to baseline levels within 4 to 6 weeks. Before the platelet peak, the percentage of reticulated platelets increased transiently in 3 patients tested, including one patient who had no response. Bleeding episodes decreased after the start of PEG-rHuMGDF therapy. These results suggest that PEG-rHuMGDF might have a clinical benefit in ameliorating thrombocytopenia associated with ITP.