Twenty-two patients with vitiligo had lesions stable for at least 1 year and 23 apparently healthy first-degree relatives were studied for T-cell subpopulations using monoclonal antibody technique. High levels of CD4+ lymphocytes and high CD4/CD8 ratios were found in both patients and their relatives, as compared with normal age-matched and sex-matched controls. This immunologic abnormality, together with the presence of autoantibodies previously reported in the apparently normal relatives, seems to be the hallmark of the disease. Further support of this interpretation comes from the observation that two out of the 23 apparently healthy relatives went on to develop vitiligo in a 2-year follow-up.