Excellent results have been reported after small caliber catheters were used for iatrogenic pneumothoraces. However, the value of using such catheters for spontaneous pneumothoraces is not clear. The purpose of this study, therefore, is to examine the efficacy of small caliber catheters in managing spontaneous pneumothoraces. Seventy six episodes of spontaneous pneumothorax were treated using a small caliber catheter (No. 5.5 or 7.0 French) connected to a Heimlich valve. All catheters were inserted by physicians. The treatment was considered successful when there was no air leakage and little or no residual pneumothorax. Before the lungs were fully expanded, four patients had died and one had refused to comply with further treatment. The remaining 71 episodes of pneumothoraces were evaluated for efficacy. The treatment was successful in 60 patients (84.5%) and ineffective in the remaining 11. The conventional large caliber tube was inserted in 10 of the 11 failures, but they were successful in only six. No major complications resulting from catheter insertion occurred, and no catheters became occluded. The catheter was easy to insert, and the scar that remained after removal of the catheter was very small. Not only are small caliber catheters effective for managing spontaneous pneumothoraces, they are the initial treatment of choice.