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The ratio of the plasma level of β-thromboglobulin (β-TG) to platelet factor 4 (PF-4) which is regarded as a most reliable indicator of platelet activation in vivo, was followed in 52 subjects at various stages of essential hypertension according to the WHO classification. These comprised 30 cases at stage 1,19 cases at stage II and three cases at stage III, and 20 age-matched normotensive control subjects. The observed p-TG:PF-4 ratio in the hypertensive patients was 4.59 ± 0.20, which was significantly higher than the value of 3.13 ± 0.19 recorded in the normotensive control subjects. According to the WHO classification, p-TG:PF-4 ratios in hypertensive patients at stages I, II and III were 3.93 ± 0.19, 5.31 ± 0.35 and 6.56 ± 0.12, respectively. The β-TG:PF-4 ratio revealed a tendency of platelet activation to increase with advanced progress of hypertensive vascular lesions.These results suggest that the abnormal platelet function observed in patients with essential hypertension plays an important role in the development of hypertensive vascular complications.