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Platelet size, expressed as mean platelet volume, was estimated in 35 patients with chronic airflow obstruction and a wide range of arterial oxygen tension (Pao2) values. In these patients there was a negative correlation between MPV and Pao2 (r = -070). Mean platelet volume was greater (9 41 (0 86) fl) in 20 patients with an arterial Pao2 of 8 kPa (60 mm Hg) or less than in 18 normal subjects (8-21 (063) fl; p < 0001). After 24 hours of supplemental oxygen treatment there was a small fall in mean platelet volume, from 9-47 (1 06) to 8-96 (0 8) fl (p < 0 05) in 12 hypoxaemic patients (Pao2 breathing air < 8 kPa) but no change in nine non-hypoxaemic patients. Larger platelets are considered to be haemostatically more active, leading to abnormal platelet function, which may contribute to the development of pulmonary vascular damage in chronic hypoxaemia. Supplemental oxygen may partially reverse these changes by modifying platelet size and activity.Platelets play an important part in thrombosis and vascular damage, but their role in the pulmonary vascular changes of patients with chronic hypoxic lung disease is uncertain. Pulmonary thromboembolism has frequently been found at necropsy in patients with chronic lung disease' and increased platelet activity could contribute to the thrombotic events. Platelet aggregation has been shown to be increased in patients with hypoxaemia and chronic airflow obstruction,2 with falls in platelet survival time, suggesting that hypoxia may increase platelet consumption.3 Evidence from work with the isolated rat lung has suggested that platelets may potentiate the hypoxic vasoconstrictor response of the pulmonary circulation, and this could contribute to the development of pulmonary hypertension.4Larger platelets, as reflected by an increased mean platelet volume, appear to be haemostatically more active, with an increased aggregating ability.56 Studies in patients with myocardial infarction have shown platelet size to be increased, and this abnormal platelet function may lead to the development of coronary thrombosis.78 In patients with chronic lung disease larger, more active platelets could contribute to the development of pulmonary vascular changes.In this study we have measured mean platelet