One hundred fifty-eight patients with active, untreated systemic lupus erythematosus (SLE) were studied from the time of diagnosis. Lymphopenia was present in 75%, and another 18% of those patients developed lymphopenia subsequent to disease reactivation. Lymphopenia of less than 1500 cells/microliter occurred more frequently than any of the preliminary criteria for the classification of SLE, and it was the most prevalent initial laboratory abnormality. Lymphocyte counts were significantly lower in lupus than in the other connective tissue diseases except mixed connective tissue disease and polymyositis.