The aim of this study was to assess the clinical, radiological and biological efficacy and tolerability of the SYSADOA, chondroitin 4- and 6-sulfate (CS, Condrosulf, IBSA, Lugano, Switzerland), in patients suffering from knee osteoarthritis. This was a 1-year, randomized, double-blind, controlled pilot study which included 42 patients of both sexes, aged 35-78 years with symptomatic knee OA. Patients were treated orally with 800 mg chondroitin sulfate (CS) per day or with a placebo (PBO) administered in identical sachets. The main outcome criteria were the degree of spontaneous joint pain and the overall mobility capacity. Secondary outcome criteria included the actual joint space measurement and the levels of biochemical markers of bone and joint metabolism. This limited study confirmed that chondroitin sulfate was well-tolerated and both significantly reduced pain and increased overall mobility capacity. Treatment with CS was also associated in a limited group of patients with a stabilization of the medial femoro-tibial joint width, measured with a digitized automatic image analyzer, whereas joint space narrowing did occur in placebo-treated patients. In addition, the metabolism of bone and joint assessed by various biochemical markers also stabilized in the CS patients whereas it was still abnormal in the PBO patients. These results confirm that oral chondroitin 4- and 6-sulfate is an effective and safe symptomatic slow-acting drug for the treatment of knee OA. In addition, CS might be able to stabilize the joint space width and to modulate bone and joint metabolism. This is the first preliminary demonstration that a SYSADOA might influence the natural course of OA in humans.