Two groups of patients, each consisting of 20 patients suffering from spinal osteoarthrosis, coxarthrosis or gonarthrosis participated in a double-blind comparative trial. They received either Lonazolac-Ca (200 mg three times daily) or Naproxen (250 mg twice daily plus 1 placebo capsule in between doses) for three weeks. The therapeutic response was assessed by monitoring pain intensity at rest and on exercise, joint tenderness and joint movement, muscular tension, morning stiffness and walking ability. In spinal osteoarthrosis vertebral flexibility was also measured. Nine out of 11 parameters showed significant improvement with Lonazolac-Ca, whereas only 6 out of 11 improved with Naproxen. However, no clinically relevant differences were recorded between both medications. Laboratory tests in both treatment groups revealed no changes in haematological or biochemical parameters nor in urinanalysis results. Drug-related gastro-intestinal side effects were observed in 2 patients of each treatment group. The patients on Lonazolac-Ca tolerated their medication with the aid of concomitant antacids whereas Naproxen had to be discontinued. One case of dermatitis, which was probably not drug-related was observed in each group. One patient suffering from migraine experienced vertigo and headache on taking Lonazolac-Ca; although this was probably not drug-related medication was discontinued.