This study compared the effects of tidal lavage (washing out) of the knee joint and an imitation lavage procedure in people with knee osteoarthritis. In tidal lavage, the doctor flushes out a knee joint with repeated injections of a mild salt solution, done under local anesthesia. Study participants had to meet standard criteria for diagnosis of osteoarthritis but could have low, medium, or high severity of x-ray changes indicating knee osteoarthritis. We performed the lavage procedure once, and did quarterly followups for 1 year. We permitted patients to use some other osteoarthritis treatments during the study, such as non-narcotic pain relievers, nonsteroidal anti-inflammatory drugs, and physical therapy.