Patients with congenital agammaglobulinemia may be separated clinically and pathologically into two groups. One group manifests a persistent lymphopenia and is characterized clinically by an unremitting course marked by pneumonia, moniliasis, and other infections usually beginning in the first three months of life and, thus far, terminating fatally during the patient's infancy. These patients have thymic alymphoplasia and lymphocytic hypoplasia of their tissues: the thymus is rudimentary, and the other lymphoid structures contain only sparse populations of small lymphocytes. The thymic alymphoplasia and general deficiency in small lymphocytes appear to be inherited characteristics and the evidence suggests that the primary defect is thymic alymphoplasia which results in generalized lymphocytic hypoplasia including lymphopenia. The second group of patients often has a later onset and a more intermittent course of bacterial infections, a variable leukocyte response marked only occasionally by transitory lymphopenia; their lymphoid organs including the thymus contain more nearly normal numbers of lymphocytes. Plasma cells and germinal follicles are absent from the tissues of both groups of patients, but the marked generalized paucity of small lymphocytes in these patients with thymic alymphoplasia results in an increased susceptibility to infection beyond that seen in nonlymphopenic agammaglobu-linemia. Whatever the exact contribution of the small lymphocyte to resistance to infection, it appears to be significant and its loss is not replaced by γ-globulin therapy.