In order to determine the spectrum and frequency of complications associated with thoracocentesis, we decided to audit prospectively all thoracocentesis performed in the medical service at our institution. Over a ten-month interval, 125 procedures were performed. We identified 114 (91 percent) prospectively, 11 retrospectively by a computer-assisted review of discharge summaries. Forty-six percent of the procedures were complicated by at least one adverse occurrence. Complications considered major occurred in 14 percent, minor in 33 percent. The major complications included 14 pneumothoraces (three required tube thoracostomies and one percutaneous aspiration), one splenic laceration, one sheared-off catheter, and one pneumohemothorax. The minor complications included pain in 28, persistent cough in 14, dry taps in 16, and subcutaneous fluid collections in four patients. We conclude that thoracocentesis can carry the risk of frequent morbidity even when a lecture and printed guidelines on performing thoracocentesis have been given and experienced individuals are in attendance during the performance of the procedure. Our study suggests a portion of this morbidity may be from poor technique, inability to adequately identify landmarks, and improper utilization of a needle-catheter apparatus. Suggestions for correction of these problems are made.