 Following a screening visit and a short baseline period to check adherence to the inclusionexclusion criteria, a total of at least 300 patients with knee osteoarthritis are randomised to receive either oral glucosamine sulfate soluble powder 1500 mg once-a-day, or acetaminophen tablets 1000 mg three times a day total 3gday, as recommended in Europe, or placebo, for 6 months. The rescue medication for flare episodes consists of the standardized use according to specific instructions given to the patients of ibuprofen 400 mg tablets, whose consumption is recorded in a patient daily diary. 