T lymphocyte regulation of immunoglobulin production may be abnormal in some patients with common variable immunodeficiency (CVI). Phenotypic analysis of peripheral blood T lymphocytes from nine patients with CVI was conducted to examine whether an abnormal distribution could be detected in a functionally distinct T lymphocyte subpopulation. The percentage of CD8+ lymphocytes proved to be increased in some patients and decreased in others. In comparison with normal controls, many patients with CVI had reduced percentages of lymphocytes expressing both CD4 and CD45RA, a phenotype associate with naive CD4+ cells. There was no significant difference in CD4+ populations bearing CD29 or leucocyte adhesion molecule-1 (LAM-1) antigens. The pattern of gene rearrangement of the T cell antigen receptor (TCR) was studied using peripheral blood lymphocytes from these patients with CVI. Genomic DNA from freshly isolated lymphocytes as well as from selectively propagated CD4+ or CD8+ populations were examined using Southern blot analysis and a probe for the beta chain of the TCR. A polyclonal pattern of TCR gene rearrangement, without the appearance of dominant non-germline bands, was demonstrated in all patient samples. These data suggest that the T lymphocytes in patients with CVI have a polyclonal pattern of TCR rearrangement despite an abnormal distribution of T cell subpopulations in some patients.