In a prospective study 26 of 29 patients with systemic lupus erythematosus had cold-reactive antilymphocyte antibodies cytotoxic for autologous lymphocytes and lymphocytes from normal subjects. The level of antilymphocyte antibodies was highly correlated, by linear regression analysis, with lymphopenia in these patients. The data suggested that both the avidity and the concentration of these antibodies were important determinants in this relationship. A clear association between increased antilymphocyte antibody activity and exacerbation of SLE was demonstrated. Apart from lymphopenia, however, neither type of clinical manifestation nor any particular serologic abnormality appeared to be related to the presence of antilymphocyte antibodies.