Analysis of platelet volume distribution curves was performed on whole blood specimens from patients with myeloproliferative disease, reactive thrombocytosis, and a control group. Estimates of the mean platelet volume and megathrombocyte index were made using either the maximum height or the area under the curve. Also, a lognormal curve was fitted to the data, providing a measure of the dispersion and another estimate of the mean platelet volume. An index expression the breadth of the distribution was derived from the ratio of megathrombocyte index to estimated mean volume. The control and reactive thrombocytosis groups were indistinguishable except for mean platelet count. The mean platelet volume and megathrombocyte index did not provide a useful separation of the myeloproliferative disease group from the other, however their ratios, when considered together with the dispersion of the distribution, enabled a distinction to be made in most cases. Thus, analysis of the platelet volume distribution is useful in detecting the presence of myeloproliferative disease.