A multi-centre, double-blind, placebo-controlled trial was carried out to investigate the clinical efficacy of the anti-inflammatory enzyme serrapeptase in a total of 174 patients who underwent Caldwell-Luc antrotomy for chronic empyema. Eighty-eight patients received 10 mg serrapeptase 3 times on the day before operation, once on the night of the operation and 3 times daily for 5 days after operation; the other 86 received placebo. Changes in buccal swelling after operation were observed as a parameter of the response to treatment. The degree of swelling in the serrapeptase-treated patients was significantly less than that in the placebo-treated patients at every point of observation after operation up to the 5th day (p less than 0.01 to p less than 0.05). Maximal swelling throughout all the post-operative points of observation was also significantly smaller in size in the serrapeptase-treated group than in the placebo-treated group. No side-effects were reported.