Purpose: Basal joint arthritis, or carpometacarpal (CMC) osteoarthritis, of the thumb is very common, particularly in the elderly. Morbidities include pain, decreased range of motion, and decreased strength. Nonsurgical treatments for CMC arthritis include oral analgesics, splinting, and steroid injection. In multiple recent trials, botulinum toxin A (BTX-A) injection has been shown to be an efficacious nonsurgical option for osteoarthritis of large joints, including those that have failed steroid injections. To our knowledge, the efficacy of BTX-A injection in thumb CMC arthritis has not been examined.