The purposes of this report are to determine the incidence of preoperative osteoarthritis in shoulders with traumatic anterior instability and to detect the factors leading to its development. The radiographs and computed tomograms of shoulders in 282 patients with unilateral instability and without previous surgery, whose age was younger than 40 years and who had no possibility of nontraumatic secondary osteoarthritis, were examined by use of a simplified modification of Samilson and Prieto's criteria. Osteoarthritis was found in 32 joints (11.3%) (mild in 30 and moderate in 2) on the radiographs. Computed tomography (CT) revealed arthritic changes in 88 shoulders (31.2%), including all 32 radiographically osteoarthritic joints. The marginal spur of the humeral head initially occurred in the anteroinferior aspect and mainly extended downward. The total number of dislocations and/or subluxations and frequency of dislocation and/or subluxation of 32 radiologically osteoarthritic joints were significantly larger and higher, respectively, than those of 194 non-osteoarthritic joints (P < .02). The total number of dislocations and/or subluxations (P < .01) and the number of subluxations (P < .05) were significantly different between osteoarthritic joints and nonarthritic joints classified by CT findings. The rate detected by CT was higher than those of any other previous radiographic survey that may have underestimated the incidence rate. Most postoperative osteoarthritis cases recognized on radiographs in short-term or midterm follow-up studies are assumed to be preoperatively generated.