This randomized, double-blind placebo-controlled parallel group study will be conducted with patients who have experienced a flare of osteoarthritis of the knee. At screening, patients whose pain is currently controlled by an analgesic will agree, after giving Informed Consent, to discontinue that analgesic. Eligible patients will be randomized (1:1 ratio) to receive double-blind treatment with either the ketoprofen topical patch or a matching placebo patch to be applied once daily for 28 days. Patients will return to the clinic on Days 7, 14, 21 and 28. At Day 28, patients may choose to open a 2 month open-label period.