The peripheral lymphocyte count and the number of large unstained cells (LUC) were investigated prior to and 4 hr after a single intravenous injection of 400 mg of hydrocortisone in 23 controls and 51 patients with lymphoid disorders (43 chronic lymphocytic leukemia, 3 cases of Waldenström macroglobulinemia, 2 hairy cell leukemias, 1 Sézary syndrome, and 2 cases of infectious mononucleosis). A reduction in both the peripheral lymphocyte counts and the number of LUC was observed in all normal controls, the mean decrease being 54% and greater than 60%, respectively, with differences according to age. In chronic lymphocytic leukemia (CLL), the peripheral lymphocyte count showed a variable response: decrease, no change, or increase. A correlation was shown to exist between a decrease in peripheral lymphocyte counts and anatomical-clinical staging: patients with involvement restricted to blood and bone marrow very often exhibited a drop in their peripheral lymphocyte count (p less than 0.01). In addition, the percentage of circulating T lymphocytes was higher (54%) in CLL patients whose peripheral lymphocyte count dropped than in other CLL patients (p less than 0.001).