Previous studies have identified a high incidence of severe combined immunodeficiency (SCID) among the Navajo Native American population. To determine the incidence and population genetics of this condition, we reviewed the death certificates of all children who died between 1969 and 1982, established the cases that met criteria identified in previously investigated cases, and interviewed the selected children's families. SCID cases were distributed spatially and temporally. Segregation parameter estimates of 0.27-0.38 were obtained from data from 24 interviewed families, suggesting an estimated gene frequency of 2.1% (arguing against a multifactorial inheritance). SCID cases referred to specialty centers lacked T and B cells in their blood, and their serum immunoglobulins ranged from absent to near normal.