In order to evaluate the clinical usefulness of carbocisteine (S-CMC) in the treatment of chronic paranasal sinusitis, a double blind trial was carried out by comparing S-CMC with L-cysteine ethylester hydrochloride (L-cysteine) as a control drug. S-CMC group (S-Group) was administered orally two tablets of S-CMC and one placebo tablet in place of active L-cysteine (1,500mg of S-CMC/day), and L-cysteine group (L-Group) one tablet of L-cysteine and two placebo tablets in place of active S-CMC (300mg of L-cysteine/day) t, i, d, after meal for four weeks. Following results were obtained: 1) The ratio of excellent or good utility of S-Group was significantly higher than that of L-Group. 2) The ratio of marked or moderate improvement of S-Group was significantly higher than that of L-Group. 3) The improvement ratio for subjective and objective symptoms of S-Group was significantly higher than that in L-Group. 4) No significant difference was found in the judgement of X-ray findings between the two groups. 5) In the respective evaluation items classified as subjective symptoms (rhinorrhea, postnasal drip, nasal obstruction) and as objective symptoms (volume of rhinorrhea, mucus rhinorrhea, postnasal drip), S-Group showed significantly superior improvement ratio in those items to L-Group. 6) Side effects were observed in two cases (1.5%) in S-Group and one case (0.8%) in L-Group. The side effects were not serious and disappeared soon after the administration was discontinued. From the above results, it is concluded that S-CMC is highly effective in the treatment of chronic paranasal sinusitis.