The efficacy and safety of SKI306X, an herbal anti-arthritic agent, was compared with that of diclofenac sodium for the treatment of osteoarthritis of the knee. In a randomized, double-blind, active comparator-controlled trial, a total of 249 patients were randomly assigned to receive either 200 mg of SKI306X three times daily or 100 mg of diclofenac sustained release (SR) once daily. Clinical efficacy variables (visual analog scale, Lesquesne index and global satisfaction score) and adverse events were monitored at baseline and 2nd and 4th weeks of treatment. SKI306X demonstrated efficacy statistically comparable to that of diclofenac, as assessed by the VAS and patients' and investigators' global satisfaction score. Both treatments were well tolerated, however, the SKI306X treatment group experienced less heartburn (4.0% versus 13.7%, p = 0.015, chi-square test). In this four-week trial, SKI306X was well tolerated and demonstrated clinical efficacy comparable to that of diclofenac SR.