A comparative study of the heterogeneity of the platelets volumes has been carried out in 9 patients with primary myeloid splenomegaly on their admission to hospital and during the evolution of their disease and in 40 healthy controls. The measurements were performed using a Coulter S-Plus system and different indexes have been calculated to better define the distribution of platelet volume. Platelet size distribution measurements demonstrated a double peculiarity in myeloid splenomegaly: an excessive microcytosis associated with a semi-linear and non log-normal distribution of macrothrombocyte volume. These data may be used as a good marker for the diagnosis of primary myeloid splenomegaly.