The aim of the study was to investigate the relevance of β-thromboglobulin (β tg) measurement in patients with thrombocytosis. We have, therefore, studied the level of plasma and platelet β tg in 74 patients with high platelet count in addition to the evaluation of platelet aggregation and platelet serotonin (5-HT) content. The determinations of platelet serotonin content and aggregation are confirmed to be useful in the differentiation between primary and secondary forms of thrombocytosis. The mean plasma level of β tg in patients with myeloproliferative disease was significantly raised, but the amount observed in subjects with secondary thrombocytosis is increased too. Considering the amount of β tg in relation to whole blood platelet count (β tg ratio), no difference was observed between all patients and controls. The β tg ratio allowed the identification of a group of patients with an increased ratio and a decreased β tg platelet content who showed the highest occurrence of thrombosis (66%).