The patient-controlled analgesia (PCA) system is the most appropriate method for the treatment of postoperative pain. Morphine is the most commonly used drug, however, its side effects and the close supervision required has driven us to search other alternative parenteral drugs among all the NSAIDs currently available. We have used metamizol, ketorolac, lysin clonixinate and propacetamol for the treatment of postoperative pain following major abdominal, gynecological, urological, ORL and traumatologic surgery with moderate or severe pain. In the 842 patients in which we have used metamizol, the results were: complete pain relief in 36% of the patients; VAS score between 2 and 4 in 54% and need of rescue analgesia in the remainder. We have used propacetamol in 31 patients following abdominal gynecological surgery with moderate to severe pain in which the 24-hour average VAS score was 3.7 +/- 3.2; 6 patients required rescue analgesia. I? our clinical trials and protocols we have obtained similar results with ketorolac and lysin clonixinate. Thus, we conclude that although most authors believe that morphine is the elective drug for the treatment of postoperative pain with PCA system, our research and clinical experience showed that metamizol, ketorolac, lysin clonixinate and propacetamol are useful for the treatment of moderate and severe postoperative pain.