To assess the efficacy of simple aspiration as a treatment for pneumothorax, 40 consecutive pneumothoraces (28 spontaneous, 12 iatrogenic, all estimated at greater than or equal to 20% collapse on visual inspection of the chest X-ray) in 38 symptomatic patients were treated initially by small-lumen catheter (SLC) aspiration. SLC aspiration avoided the need for large-lumen intercostal catheter (LIC) underwater drainage in 28 cases (70%)--20 of 28 spontaneous and eight of 12 iatrogenic pneumothoraces. Outcome was not predicted by clinical variables or pneumothorax size, whereas an initial aspirate volume of less than or equal to 4 L (n = 33) was predictable of success in 28 cases (85%). Minor local subcutaneous emphysema and vasovagal reactions were encountered infrequently but with similar frequency to LIC drainage. No episodes of re-expansion pulmonary oedema occurred. The results confirm previous reports of the efficacy of simple aspiration as a treatment for spontaneous or iatrogenic pneumothorax. Initial treatment by SLC aspiration is recommended for all but life-threatening presentations of pneumothorax. Although not encountered in this study, the potential risk of re-expansion pulmonary oedema suggests that patients should be observed closely for four hours after aspiration.