The aim of the study was to determine the incidence and etiology of thrombocytosis in an adult Turkish population. The medical records of 2000 patients were reviewed retrospectively to determine (i) the etiology of elevated platelet counts, (ii) additional laboratory parameters (i.e., complete blood counts), and (iii) the presence of thromboembolic complications. Within 5 years, 174 678 platelet counts had been performed for 124 340 patients. Of the 124 340 patients, 2000 (1.6%; mean age, 55.2 +/- 17.4 years; range 16-94 years; male-to-female ratio, 3.6:1 [ 1570: 430]) had one or more platelet counts of 500x10(3)/mm(3) or higher during the study period. Secondary thrombocytosis was the most frequent cause of an elevated platelet count and occurred in 1934 cases (96.7%). Infection was the most common cause of secondary thrombocytosis, occurring in 1001 patients (50.1%). Compared with secondary thrombocytosis, primary thrombocytosis was significantly associated with a higher rate of thromboembolic complications. It is concluded that a high platelet count may have both diagnostic and therapeutic implications. Secondary thrombocytosis is a common finding and mostly secondary to infection. Primary thrombocytosis has a higher risk for thromboembolic events than does secondary thrombocytosis.