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Gaseous exchange and regional pulmonary function (using 133Xe) were studied in 10 subjects with hemidiaphragmatic paralysis; the paralysis was presumably idiopathic in nine subjects. Global and regional investigations were performed on the same morning, the subjects being studied in the sitting position. Six subjects were obese; nevertheless overweight was moderate except in one case. Lung volumes were moderately decreased (vital capacity ranged from 69 to 100 percent of the predicted value). PaO2 was normal or nearly normal: mean = 75.8 +/- 7.5 torr in the sitting position. PaO2 did not vary significantly in the supine, nor in the lateral recumbent position. AaDO2, breathing air or during hyperoxia, was slightly increased. Qs/Qt was markedly increased in only three cases. Regional lung function data suggest that the paralysed side still plays an appreciable part in pulmonary volume (43.5 +/- 6.3 percent), ventilation (39.1 +/- 8.2 percent) and perfusion (40.4 +/- 9.3 percent). Impairment of the paralysed side was mainly localized in the lower zone; in the middle zone only ventilation was significantly reduced. Results of regional ventilation and perfusion, expressed as absolute values, suggest a redistribution of ventilation and perfusion from the base towards the apex, not only for the paralysed side but also for the opposite side. This adjustment is probably due to the hemidiaphragmatic paralysis but the possible role of obesity (present in six cases) must be kept in mind. Possible therapeutic deductions are discussed.