Fourteen paediatric patients with advanced amegakaryocytic thrombocytopenia (AMT) or other bone marrow (BM) failure syndromes were enrolled on one of two phase I/II dose escalation studies of PIXY321. PIXY321 was administered subcutaneously in doses ranging from 250 to 750 mg/m2/d. No dose-limiting toxicity was observed. Peak absolute neutrophil count (ANC) was higher than baseline in all patients. Most transfusion-independent patients demonstrated elevation in haematocrit and/or platelet count. Trilineage haemopoietic responsiveness was evident in the three transfusion-independent patients. In these paediatric populations PIXY321 is well tolerated and merits consideration as a potential therapy.