Abstract Botulinum toxin type A (Btx-A) has been extensively studied and used clinically for its muscle paralyzing effects, but there is a growing body of evidence to support a role in pain modulation. Symptomatic osteoarthritis is a leading cause of pain, functional impairment, and disability, with significant indirect costs to society. Preliminary evidence suggests that Btx-A has a significant nociceptive effect, when injected intra-articularly, in to painful joints. The proposed study will evaluate the efficacy and safety of Btx-A injected intra-articularly in 60 subjects with moderate pain and functional impairment secondary to knee osteoarthritis, in a randomized, cortisone-controlled, double blind study, over a 6 month follow up period. If Btx-A is shown to be of equal or greater efficacy than cortisone in this patient population, it may be an excellent second line treatment for osteoarthritis, in multiple joints, where surgery is contraindicated or deferred due to age, comorbidities, or patient preference. Further studies examining the mechanism of action at the biochemical level, the clinical effect of Btx-A in other joints (in both osteoarthritis and inflammatory arthritis), the efficacy of Btx-A compared to hyaluronic acid (the only currently available injectable alternative to cortisone), and the side effect profile (effect on adjacent muscle strength, joint position sense, and long-term outcomes) would be indicated.