This study aimed to evaluate whether computed tomography improves the intra- and inter-observer reliability of the Eaton-Glickel classification of trapeziometacarpal joint osteoarthritis. The osteoarthritis of the trapeziometacarpal joint was evaluated with conventional radiographs and computed tomography by two hand surgeons, two registrars and one radiologist in 50 hands of 43 patients (12 male and 31 female) who had a median age of 60 years (46-80). Using plain radiographs, we found a mean intra-observer reliability of 0.54 (0.51-0.74), and the intra-observer reliability was improved to 0.76 (0.64-0.86) using computed tomography. Using plain radiographs, the mean inter-observer reliability was poor at 0.17 (0.04-0.51) and did not improve using computed tomography with a mean inter-observer reliability of 0.22 (0.02-0.38). In particular, the agreement in the distinction between Eaton-Glickel stage III and IV and the evaluation of the degenerative changes in the scaphotrapezio joint was low using computed tomography-scans. The detection of bone cysts on computed tomography was more precise than on plain radiographs.