Objective. To assess the safety and efficacy of pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF) administered after autologous bone marrow transplantation (ABMT). Patients and Methods. Two randomized, double-blind, placebo-controlled studies were done. In the phase 1/2 study, 75 breast cancer patients underwent a bone marrow harvest and myeloablative STAMP V chemotherapy and were randomized to receive placebo or one of three doses of PEG-rHuMGDF. In the phase 3 study, 64 patients were randomized to receive placebo or the minimally effective dose of PEG-rHuMGDF. The study drug was administered daily starting on the day of bone marrow infusion until the platelet count was greater than or equal to 50 × 109/L (without transfusion) or for a maximum of 28 days. All patients received 10 μg/kg/day filgrastim starting on day 2 until neutrophil count recovery. Results. PEG-rHuMGDF appeared to be safe and well tolerated. No significant differences were noted in mortality or disease progression rates. Antibodies to MGDF were not observed. In the phase 1/2 study, the time to platelet recovery to greater than or equal to 20 × 109/L and platelet transfusion requirements were significantly reduced for patients treated with PEG-rHuMGDF compared with placebo (p < 0.05). In the phase 3 study, no significant differences in the kinetics of early thrombopoiesis or platelet transfusions after ABMT were observed. Conclusion. PEG-rHuMGDF was not consistently efficacious in reducing the duration of severe thrombocytopenia. The maximum platelet counts for PEG-rHuMGDF-treated patients occurred a median of 2 weeks after the last dose of drug, suggesting that the biologic effects of this hematopoietic cytokine are delayed compared with other hematopoietic cytokines.