The study objective is the eluciation of the dose-response function in efficacy (and possibly safety) in order to determine an optimal dose with respect to efficacy and safety. The study is designed with three dose levels and one placebo control.

Inclusion Criteria:

• clinical diagnosis of OA in at least one knee for a minimum of 6 months
• aged 18-75 years old
• eligible subjects must have used an oral or rectal NSAID on at least three days per week during the three months before screening or on at least 25 of the 30 days before screening, but be dissatisfied with the current NSAID treatment
• at baseline visit WOMAC pain subscale at least 40 mm, and at least 15 mm greater than at the screening visit in the index knee
• radiographic evidence consistent with OA criteria of grade 2 or 3 according to Kellgren & Lawrence in the index knee. Radiographs mus have been taken during the 6 months before baseline.
• American College of Rheumatology (ACR) functional class I, II or III

Exclusion Criteria:

• intraarticular injections or arthroscopy of the index knee within 3 months prior to screening, or planned during the study
• history, physical examinations or radiographs suggestive of other rhematic diseases
• known hypersensitivity or contraindication to ketoprofen; NSAID idiosyncrasy
• history of peptic ulcers
• severe liver or gastro-intestinal disease within the last 6 months
• injections or change in oral treatment regimen of glucosamine, chondroitin sulfate, hyaluronic acid, shark cartilage, or vitamin E within 3 months prior to screening
• oral or parenteral corticosteroids within 3 months prior to screening
• analgesic therapy for chronic or recurrent pain conditions for indications other than OA
• immunosuppressants within 3 months prior to screening