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In an attempt to understand the mechanism underlying the relief of dyspnea that follows thoracocentesis in patients with large pleural effusions, we measured respiratory mechanics in nine patients before and two hours after removal of 600 to 2,750 ml (mean = 1,818 ml) of pleural fluid. Thoracocentesis resulted in only small changes in pulmonary mechanics: Mean vital capacity and functional residual capacity increased by 300 and 460 ml, respectively, lung recoil pressure slightly decreased, and mean static expiratory compliance increased by 0.021 liter/cm H₂O. These changes were inconsistent and could not explain the immediate and remarkable relief of dyspnea noted by the patients. By contrast, thoracocentesis invariably resulted In a shift of the minimal (inspiratory) pleural pressure-volume curve so that the pressures generated by the inspiratory muscles were markedly more negative at any comparable lung …