The hematologic and histologic features of two, nontwin, male siblings with severe combined immunodeficiency and variable granulocytopenia are compared to the four previously reported cases of reticular dysgenesis. These sibs died at 50 and 3 days of age, respectively, with Pseudomonas sepsis and congenital cytomegalovirus infection, respectively. A maternal uncle has selective IgA deficiency. Cord blood from the second sib contained a normal percentage of E-rosetting lymphocytes; however, these lymphocytes failed to respond to mitogenic stimulation in vitro. Erythrocyte and lymphocyte levels of adenosine deaminase were elevated in the father and the second sib. Serum immunoglobulin concentrations were low in both siblings.