Erythrocyte properties of patients with congenital hypoplastic anemia were compared to those of patients with transient erythroblastopenia of childhood. The MCV was less than 85 in all nine TEC patients studied and greater than 90 in all 11 CHA patients. Hemoglobin F concentration was elevated beyond the normal level for age in eight CHA patients and almost always normal in TEC. The i antigen score was more likely to be elevated in CHA than in TEC. The activities of transaminase, aldolase, phosphofructokinase, and glutathione peroxidase were higher in CHA than in TEC (p less than 0.001). Some abnormal properties (namely, MCV and hemoglobi n F concentration) of CHA erythrocytes, present during remission but accentuated during relapse, seemed to vary with changes in serum erythropoietin. Early differentiation of TEC and CHA appears feasible, allowing prompt provision of a favorable prognosis and the avoidance of unnecessary corticosteroid therapy in TEC.