The efficacy and safety of ademetionine (A) vs naproxen (N) were tested in a double-blind trial carried out in 20 patients, each with activated gonarthrosis. The trial lasted 6 weeks. During the first week, A was administered at a daily dose of 3 x 400 mg and afterwards at a dose of 2 x 400 mg, whereas the daily dose of N during the first week was 3 x 250 mg and subsequently 2 x 250 mg. During the first two weeks, the patients were allowed to take paracetamol as an additional analgesic. The patients were examined at the beginning of the study and after 2, 4 and 6 weeks. The parameters tested were: pain (under different conditions), crepitation, joint swelling, circumference of joint, extent of motility and walking time over 10 meters. In addition to the usual laboratory tests, the serum keratane-sulphate concentrations (with monoclonal antibodies according to the ELISA technique of Eugene et al. [1985]) were also determined. At the end of the 6th week no statistically significant difference between the two patient groups treated was found; both groups exhibited a marked improvement on all parameters. At the end of medication, the keratane-sulphate concentrations were not significantly changed. Five patients under A and 3 under N reported gastrointestinal side effects which were possibly drug-related. This study, performed in a small number of patients, showed a good efficacy and safety of ademetionine. Only further studies on a larger scale will show the importance of ademetionine in the therapy of rheumatic diseases.