There is no documentation in the literature of the risk of hypoxemia after chest percussion in acutely ill adults with nonsurgical pulmonary disorders. We studied the immediate effect of postural drainage and chest percussion (PDP) on oxygenation in 22 hospitalized patients with a variety of acute, nonsurgical pulmonary disorders. Heart rate, respiratory rate, blood pressure, and arterial blood gas levels were measured at four points before, during, and after PDP. There was a significant fall in PaO2 after chest percussion in ten patients who produced no sputum or small amounts of mucoid sputum. The mean PaO2 fell 16.8 mm Hg (P < 0.05) immediately after PDP, when compared to the value obtained after postural drainage alone, and had fallen another 5.3 mm Hg (P < 0.01) 30 minutes after return to the pretreatment position. There was no significant change in PaO2 in 12 patients who produced moderate to large amounts of mucopurulent secretions. The fall in PaO2 was probably due to increased ventilation-perfusion mismatch since this fall was avoided in two patients restudied while receiving 100 percent oxygen. We concluded that all acutely ill patients receiving PDP should be carefully monitored and, if necessary, should receive increased levels of inspired oxygen to avoid hypoxemia. Our data suggest that the use of PDP in patients without sputum production is not indicated and is potentially dangerous.