Yesterday's immunodeficiencies emphasized the clinical and familial associations of the syndromes and date from the 1920s (ataxia-telangiectasia, chronic mucocutaneous candidiasis), the 1930s (Wiskott-Aldrich syndrome), skipping the 1940s, but blossoming in the 15-y period from 1950 to 1965. In this period, primary immunodeficiencies affecting all the major limbs of the immune system were first described (1950: severe combined immunodeficiency; 1952: X-linked agammaglobulinemia; 1957: chronic granulomatous disease; 1965: C2 deficiency). Today's immunodeficiencies, as detailed in Stiehm's Immunologic Disorders in Infants and Children (Edition 1, 1973; Edition 2, 1980; and Edition 3, 1989) emphasize the immunologic and genetic aspects of immunodeficiency. These increased from 43 syndromes in the 1973 edition (34 primary, nine secondary) to 94 syndromes in the 1989 edition (66 primary, 28 secondary). This means that about two primary and one secondary immunodeficiencies have been uncovered annually. Tomorrow's immunodeficiencies, to be covered in Edition 4, will include new clinical and immunologic observations and molecular and biochemical studies that characterize some unique immunodeficiencies. These include the following six groups of defects: 1) neutropenic syndromes with hypogammaglobulinemia, including the WHIM syndrome; 2) phenotypic genetic syndromes with immunodeficiency including Bloom's syndrome and Schimke's immuno-osseous dysplasia; 3) natural killer cell defects associated with a) other primary immunodeficiencies, b) other nonimmunologic illness, and c) primary natural killer defects; 4) T-cell membrane defects; 5) IL defects; and 6) miscellaneous phagocytic illnesses including periodontitis and the asplenia syndrome.