Patient-controlled analgesia (PCA) is one of the more popular means of controlling postoperative pain. However, there is very little in the literature concerning the adverse outcome of respiratory depression in PCA. This report is a prospective study of 4,000 patients on PCA postoperatively. Nine of these patients experienced respiratory problems while on PCA. The respiratory depressions were associated with drug interactions, continuous narcotic infusion, nurse- or physician-controlled analgesia and inappropriate use of PCA by patients. This report identified the common precipitating factors in PCA-associated respiratory depression and its prevention.