The definition of the congenital immunodeficiency syndrome (CIDS) is given. Structural alterations of the thymus in combination with zonal disturbances and cell composition of the peripheral lymph organs are main morphological manifestations of CIDS. The most reasonable classification of CIDS is that based on etiology and thus making necessary to reconsider the existing terminology with an exclusion of such terms as "primary" and "secondary" immunodeficiency. The crucial point in the CIDS pathogenesis is a time of structural disturbances resulting from the action of an altering factor on the immune system in ontogenesis. A list of diseases of the histiomonocytic system is given which can be used as a basis for developing the classification of CIDS of the above system as the most ancient element in the immune response in the phylo- and ontogenesis.