B and T cell phenotypes in peripheral blood from 71 CVID patients have been measured in a study using directly conjugated monoclonal antibodies and two colour flow cytometry. Data was compared between different patient groups (based on whether their B cells could secrete IgM or IgG in vitro) and normal donors. There was a clear correlation between abnormalities of both B and T cells and the different patient groups. There were reduced absolute numbers of circulating CD4+ T cells, particularly those of the CD4+.CD45RA+ subset, and of CD19+ B cells in those patients whose B cells failed to secrete IgM or IgG in vitro. This demonstrates an association between B cell lymphopenia, failure of B cell immunoglobulin production in vitro and T cell subset lymphopenia in CVID. It supports the view that this group of CVID patients has a disease involving T cell regulation of B cells of varying severity.