Varying opinions exist regarding the choice of initial treatment for primary spontaneous pneumothorax (PSP). This study aims to determine the success rate of simple aspiration of PSP, identify factors associated with failure, and identify associated complications. A retrospective analysis was performed on 91 consecutive patients receiving simple aspiration for PSP at the Emergency Department of a university teaching hospital in Hong Kong, China. The overall success rate was 50.5%. Failed cases had significantly larger sizes of pneumothorax, and significantly larger volumes of air aspirated. Pneumothorax size > or =40% was significantly associated with failure. In a multivariate analysis, pneumothorax size > or =40%, compared to size 21-39%, independently predicted failure, with an odds ratio of 8.88 (95% CI, 2.49 to 31.63). Complications were rarely encountered. It is concluded that the success rate may be significantly improved by excluding patients with pneumothorax size 40% or larger from this modality of treatment.