In ataxia-telangiectasia, B-cell and T-cell deficiencies are thought to be due to a defect of rearrangements of immunoglobulin and T-cell receptor genes. T cells recognize antigens through two types of CD3-associated receptors: alpha/beta chains on mature cells and gamma/delta chains mostly on immature cells. We studied 10 patients with ataxia-telangiectasia and found that most had a relative increase of circulating T cells bearing gamma/delta receptors rather than alpha/beta receptors, as compared with normal subjects (P less than 0.001). Patients with other immune deficits, including eight with common variable immunodeficiency, one with Wiskott-Aldrich syndrome, two with hyperimmunoglobulinemia E syndrome, and one with severe combined immunodeficiency, had normal ratios of gamma/delta-bearing to alpha/beta-bearing cells. A marked predominance of gamma/delta-bearing T cells was found in a patient with a primary T-cell defect. The relative increase in gamma/delta-bearing T cells in the patients with ataxia-telangiectasia was largely accounted for by cells that reacted with the monoclonal antibody BB3, an apparently distinct subset of T cells that selectively express the C gamma 1 gene product of the T-cell receptor. Although they had normal ratios of gamma/delta-bearing to alpha/beta-bearing T cells, the patients with common variable immunodeficiency had a significant increase (P = 0.01) in the number of T cells expressing C gamma 2 that reacted with the monoclonal antibody delta-TCS-1. We conclude that the increased ratio of gamma/delta-bearing to alpha/beta-bearing T cells in ataxia-telangiectasia may reflect both a recombinational defect that interferes with T-cell and B-cell gene rearrangements and an inability to repair damage to the DNA.