The use of electronic pulse spectroscopy to determine the volume distribution of a white blood cell fraction isolated by sedimentation from anticoagulated whole blood permits a rapid quantitative measure of size and dispersion of white blood cells in suspension. In patients without apparent hematologic diseases the white cell volume distribution presents as a broad peak. In patients with a large percentage of circulatory malignant cells, a single narrow volume peak is observed in acute leukemias and a broad and sometimes bimodal curve is observed in chronic leukemias. The volume of the leukemic cells is observed to shift in vivo after the start of effective chemotherapy as was predicted on the basis of a tissue culture model. The prognostic significance of the white cell volume distribution in leukemia and the use of a shift in size of leukemic cells after the start of therapy as a means of monitoring therapy are discussed.