The purpose of this study is to confirm the efficacy and safety of 800 mg Chondroitin 4&6 sulfate (Condrosulf) vs placebo once a day for 6 months in the symptomatic treatment of knee osteoarthritis. A third group, Celecoxib 200 mg (Celebrex) once a day, will be used as active comparator.

Inclusion Criteria:

• Outpatients of either sex, aged ≥50 years
• Patients affected by primary knee osteoarthritis of the medial or lateral femoro-tibial compartment
• Diagnosis according to the American College of Rheumatology (ACR) criteria
• Kellgren & Lawrence grade I-III
• Knee osteoarthritis evolving for more than 6 months
• Patients suffering from regular pain and functional disorders from at least 3 months
• Accomplishing a score ≥ 7 of Lequesne's index for the knee osteoarthritis
• Assessing pain on Huskisson's VAS ≥ 50 mm
• With radiography dated less than six months showing a remaining articular joint space
• Without such an axial disorder to justify an osteotomy
• Women taking contraceptive measures if not in menopause
• Women having negative pregnancy test
• Patients able to understand and follow the study protocol
• Patients who have signed the written informed consent for their participation in the clinical trial

Exclusion Criteria:

• With a history of heart attack, ischemic heart disease or cerebrovascular disease (including transient ischemic attacks)
• Having or have had peripheral arterial disease or past surgery orf peripheral arteries
• With a history or currently significat coagulation defect or/and blood dyscrasia
• With high risk of cardiovascular events
• With any acute or chronic infections requiring antimicrobial therapy or serious viral (e.g., hepatitis, HIV positivity) or fungal infections
• With a history of recurrent gastrointestinal ulceration or active inflammatory bowel diseases (e.g., Crohn's disease or ulcerative colitis)
• Having been diagnosed as having or have been treated for oesophageal, gastric, pyloric channel, or duodenal ulceration within 30 days prior to receiving the first dose of study medication
• Having severe liver or kidney disease
• With allergy to Celebrex or any of the other ingredients of Celebrex
• Having had an allergic reaction to sulphonamides
• Having had, as a result of intake of acetylsalicylic acid or other NSAIDs, asthma, nose polyps, severe nose congestion, or an allergic reaction such as itchy skin rash, swelling of the face, lips, tongue or throat, breathing difficulties or wheezing
• Presenting lactose intolerance
• Mild or not symptomatic knee osteoarthritis : < 7 of Lequesne's index,
• Pain on Huskisson's VAS (Visual Analogic Scale) < 50 mm
• Predominantly femoro-patellar osteoarthritis
• Destructive osteoarthritis of the knee justifying a surgery in the following 6 months
• Osteoarthritis with hydrarthrosis requiring a puncture or an infiltration
• Important genu varum or valgus >8° (physiological angle including)
• Kellgren & Lawrence grade IV
• Knee joint surgery in the last 3 months (e.g. chondroscopy, arthroscopy)
• Viscosupplementation, tidal lavage in the last 6 months
• Arthritis and metabolic arthropathies, Paget's illness
• Having consumed: basic treatment of arthritis with SYSADOA, symptomatic slow acting drugs for osteoarthrithis (chondroitin sulphates, glucosamine sulphates, diacerhein, hyaluronic acid and food supplement for joint care) in the last 3 months; treatment with corticoids, by any administration route during the last month; any medication having an influence on pain:
• NSAIDs (nonsteroidal anti-inflammatory drugs) in the 5 days preceding the inclusion (wash-out period considering 5 half-lives of the drug)
• hypnotics, muscle relaxants, anxiolytics, if the intake has started less than 8 days before inclusion
• paracetamol in the 10 hours preceding the inclusion
• Foreseen physiotherapy, re-education, alternative medicine (mesotherapy, acupuncture) in the next six months (study period)
• Presenting psychiatric illness hindering the protocol complaince, alcoholism, ongoing or < 1 year drug dependency
• Pregnant or likely to become it during clinical trial or lactating
• Women having positive pregnancy test
• Having participated in other clinical trials in the month preceding the clinical trial.