The prevalence of specific thrombotic accidents recognized in 260 patients with thrombocytosis are reported. Ninety one were affected by PV, 86 by ET, 20 by MF and 63 by ST. The highest incidence of thrombosis was in the PV group. About half of ET and MF patients experienced a thrombosis. In all the patients thrombosis preferentially affected the cerebrovascular district (17.3% of the cases). Coronary artery disease occurred in PV while peripheral vascular disease was frequent in ET. Portal vein district thrombosis is not rare during the course of all MPD (7.1%). Apparently, deep vein thrombosis occurs in all patients with both primary and secondary thrombocytosis.