Congenital amegakaryocytic thrombocytopenia (CAMT) is an uncommon disorder in newborns and infants, characterized by isolated thrombocytopenia and megakaryocytopenia in the first year without physical anomalies. The defect of thrombopoiesis is not well understood. Recently, thrombopoietin (TPO), the ligand for the c‐mpl receptor, was cloned. Accumulating evidence from in vitro and in vivo studies indicate that TPO plays a key role in the regulation of megakaryocytopoiesis. In this study we examined the effect of TPO on megakaryocyte colony formation from a patient with CAMT using a plasma‐containing methylcellulose clonal culture. The in vitro results demonstrated a defective response to TPO in megakaryocyte colony formation from bone marrow mononuclear cells (MNC) of the patient, although interleukin‐3 (IL‐3) but not stem cell factor (SCF) induced only a small number of megakaryocyte colonies. These findings indicated that thrombocytopenia in CAMT could not be corrected by administration of TPO in vitro. Additionally, clonal cultures containing SCF, IL‐3, IL‐6 and erythropoietin showed decreased numbers of erythroid and myelocytic progenitors in the bone marrow of the patient. The serum TPO level measured by enzyme‐linked immunosorbent assay was significantly higher than that in healthy controls. By PCR, marrow MNC from healthy children and from a patient with essential thrombocytosis expressed c‐mpl mRNA, whereas no c‐mpl mRNA was detected in marrow MNC from the patient with CAMT. There was no difference in the CD34 expression and c‐kit mRNA between the CAMT patient and healthy children. The results of this study suggest that the pathophysiology in CAMT may be a defective response to TPO in haemopoietic cells through impaired expression of c‐mpl mRNA.