Forty examples (27 from the literature and 13 new cases) of a syndrome of hypomegakaryocytic thrombocytopenia with bilateral absence of the radius have been analyzed. This syndrome is designated in this paper as "thrombocytopenia with absent radius (TAR)". The onset of hematologic complications usually occurs at birth or during early infancy. Thrombocytopenia may be episodic and sometimes is accompanied by leukemoid reactions and eosinophilia. Bone marrow examination reveals decreased and/or abnormal megakaryocytes, with normal myeloid and erythroid precursors. Congenital skeletal deformities include bilateral absence of radius, shortening and deformity of the ulnae, and occasionally absence of all the long bones in the arm. The fingers and thumbs are always present. Other skeletal anomalies are frequent. Cardiac anomalies, particularly the tetralogy of Fallot and atrial septal defects, may be present. Other non-skeletal congenital abnormalities are rare. The prognosis is good if the patient survives to one year of age. The syndrome has been compared to Fanconi's anemia, thalidomide embryopathy, limb-cardiovascular syndrome, and a syndrome of multiple congenital malformations, from which it can be distinguished.