To assess whether pleural pressure (PP) measurement is helpful in evaluating the evolution of spontaneous pneumothorax (SP). To measure the inspiratory and expiratory PP at tidal volume in the SP in 85 cases. Ninety-one percent were cured with medical treatment, whereas the remaining 9% required surgery. In the cases medically resolved, the inspiratory pleural pressure (IPP) was -9 +/- 5 mbar, and the expiratory pleural pressure (EPP) was -3 +/- 6 mbar, whereas in the surgical cases, IPP was -7 +/- 2 mbar (p = not significant), and EPP was 1 +/- 3 mbar (p < 0.01). In the SP cases, which resolved in less than 7 days of drainage, IPP was -10 +/- 5 mbar, and EPP was -3 +/- 5 mbar, whereas IPP was -8 +/- 3 mbar (p < 0.01), and EPP was 0 +/- 4 mbar (p < 0.001) in those cases in which SP resolution required more than 7 days. The sensitivity to predict the need for surgery with an EPP > or = 2 mbar was 100%, with a specificity of 49%. The cutoff of the curve was > or = 1 mbar, with a sensitivity of 65% and specificity of 70%. The values of PP at the end of an expiration to tidal volume were negative in the SP cases, which were resolved by medical treatment, whereas they were atmospheric in those cases that required surgery. The static inspiratory and expiratory PP in the SP cases requiring thoracic drainage during < 7 days showed more negative PP than those that resolved in a period longer than 7 days.