Cell surface CD8 molecules exist as either alpha/alpha homodimers or alpha/beta heterodimers, and most CD8+ T cells that undergo differentiation in the thymus express alpha/beta heterodimers. CD8 molecules on peripheral blood lymphocytes of patients with Wiskott-Aldrich syndrome (WAS), an X-linked immunodeficiency disease, have now been shown to be composed mostly of alpha/alpha homodimers, while the total number of lymphocytes was maintained. This observation may indicate aberrant differentiation of CD8+ T cells in the thymus of WAS. Decreased CD8alpha/beta heterodimer expression, and the relative increase of CD8+ T cells expressing CD8alpha/alpha homodimer, may explain a part of the defective lymphocyte functions in WAS patients.