A case of hereditary incomplete, combined immunodeficiency is reported. The patient, a 20-week-old boy, suffered from severe, generalized tuberculosis after BCG-vaccination. A conspicuous discrepancy was found between the normal number of lymphocytes in the peripheral blood and the severe morphological changes in the organs of the lymphoreticular system (deficient development of thymic structures; absence of lymphocytes in the thymus and the thymus-dependent areas of the periphery as well as in the bursa-dependent structures). A partial primary defect of the stem cells and a secondary insufficiency of the functions with incomplete differentiation due to an insufficiency of the primary lymphatic organs are discussed as possible causative factors. The inability to develop epitheloid cells in connection with the tuberculous infection is interpreted in part as a sequence of a T-cell insufficiency.