Thrombocytopenia with absent radii (TAR) syndrome is a rare congenital defect with severe hypomegakaryocytic thrombocytopenia and bilateral radial aplasia. To elucidate a possible relationship between thrombocytopenia in TAR and defects in the thrombopoietin (TPO)/c-Mpl system, we examined TPO activity in sera from six patients and in vitro reactivity of the patients' platelets to recombinant human TPO. We found elevated TPO serum levels in all patients, excluding a TPO production defect as a pathomechanism for the thrombocytopenia. In contrast to healthy controls, however, platelets of TAR patients failed to respond to recombinant TPO as measured by testing TPO synergism to suboptimal concentration of platelet activators. Most interestingly, TPO-induced tyrosine phosphorylation of platelet proteins was completely absent (four out of five) or markedly decreased (one out of five). More detailed investigations of the signal cascades of c-Mpl demonstrated the absence of Jak2 phosphorylation after TPO stimulation in a TAR patient's platelets. A defect in the early events of c-Mpl signal transduction might be the reason for impaired megakaryocytopoiesis in TAR syndrome.