Oxygen arterial partial pressure (PaO2) has been measured in 38 patients with idiopathic spontaneous pneumothorax (ISP) and in 20 of them 8-9 days after full expansion of the lung within 1-3 days by aspiration through chest tube drain. The PaO2 was initially 69.0 +/- 9.6 Torr (predicted value 99.6 +/- 6.6 Torr, p < 0.01) and the alveolo-arterial O2 pressure (AaDO2) difference was 28.2 +/- 8.9 Torr (pred. val. 8.6 +/- 2.5 Torr, p < 0.05). PaO2 in complete ISP (25 cases) was 65.6 +/- 6.9 Torr and in incomplete ISP 74.3 +/- 11.1 Torr (p < 0.01). PaO2 difference between initial values (64.6 +/- 7.4 Torr) and after full expansion of the lung (77.6 +/- 10.0 Torr), as measured in 20 cases and AaDO2 difference (27.5 +/- 9.8 Torr and 15.33 +/- 10.5 Torr), were statistically significant with p < 0.01). The hypoxemia still observed after full expansion of the lung may be explained by the persistent small airway closure possibly due to increased surface tension and the subsequent transsudation still present in some peripheral air spaces.