The effect of induced hypothyroidism on platelet count and platelet volume distribution was studied in twelve athyreotic patients. After a two weeks withdrawal of triiodothyronine supplementation, platelet count and the ratio between platelet and red cell count were increased in all patients. Furthermore, mean platelet volume was declined and platelet distribution width was risen. Thus, hypothyroidism appears to increase the number of circulating platelets, especially the smaller ones.