Abstract: We measured the calmodulin content in platelets in 13 normal persons and in 62 patients with hematological diseases. The level of platelet calmodulin was higher in patients with idiopathic thrombocytopenic purpura (ITP), systemic lupus erythematosus, myeloproliferative disorders, acute leukemia in a recovery phase, aplastic anemia, thrombosis and hyper‐splenism as compared to the controls. Among the patients with ITP, calmodulin was lower in responders than in nonresponders and those at the initial diagnosis. We also measured the volume, life‐span and aggregation of the platelets and demonstrated a significant relationship between the calmodulin level and the platelet volume, and a negative relationship between the calmodulin level and platelet life‐span, there was no correlation between the calmodulin level and platelet aggregation. We thus conclude that platelet calmodulin is inversely correlated with platelet turnover.