Computerized tomography was done preoperatively on twenty shoulders (thirteen patients) in which there were severe arthritic changes, to measure glenoid version. Ten of the twenty shoulders had osteoarthrosis; eight, rheumatoid arthritis; and two, gouty arthritis. To help determine normal values, computerized tomographic scans of the chest of sixty-three patients who did not have roentgenographic evidence of disease of the shoulder were studied retrospectively for comparison as a control group. In the group of patients who had severe arthritis, the mean glenoid orientation was 11 degrees of retroversion (range, 2 degrees of anteversion to 32 degrees of retroversion). The computerized tomographic scans showed uneven wear of the glenoid surface, osteophytes, large cysts, and posterior displacement of the humeral head. In the control group, the mean orientation of the glenoid was 2 degrees of anteversion (range, 14 degrees of anteversion to 12 degrees of retroversion). The difference between the groups was significant (p less than 0.0001). Glenoid retroversion was increased in the patients who had severe arthritis, and the computerized tomographic scans accurately revealed the extent and pattern of erosion of the bone.