Seven cases of total shoulder arthroplasty exhibiting major glenoid radiolucent lines or actual translation of the glenoid component were evaluated to identify factors associated with glenoid loosening. The average time from arthroplasty was 30 months (range, 14-44 months). Six of the patients had severe, incompletely reconstructable rotator cuff tears present at the time of surgery, and one patient developed a cuff tear within 1 year of surgery. The amount of superior migration of the humeral component was closely correlated with the degree of glenoid loosening. With superior displacement of the humeral component, superior tipping of the glenoid component was observed: a "rocking horse" glenoid. For comparison, a contemporary group of 16 consecutive total shoulder arthroplasty patients with intact rotator cuffs were reviewed. The control group had no glenoid loosening an average of 5 years after operation. Upward riding of the prosthetic humeral head in patients with rotator cuff deficiency may contribute to loosening of the glenoid component in total shoulder arthroplasty.