Peripheral blood lymphocyte size distribution curves in normal children and adults revealed a very constant pattern with a modal class size of 9.5μ to 10.4μ and a curve slightly skewed to the right. In acute infections characterized by abnormal lymphocytes an irregular alteration in the size distribution curve was seen. This pattern returned to normal following recovery. In children with immunologic defects abnormal distribution curves were seen. These abnormalities were unrelated to absolute lymphocyte count or to the clinical condition at the time measurements were determined. Patients with combined immunodeficiency states showed a biphasic abnormality with two distinct peaks at 13μ and 15μ. This was most pronounced in ten patients with the Wiskott-Aldrich syndrome. In one of these patients the curve became normal following successful bone marrow transplantation.