BACKGROUND Proximal humeral migration is commonly seen in rotator-cuff-deficient shoulders. The specific effects of the size of the rotator cuff tear and of pain on glenohumeral kinematics have been poorly defined. The purpose of this study was to examine the influences of cuff tear size and pain, separately, on humeral migration in a series of patients with symptomatic and asymptomatic rotator cuff tears. METHODS Ninety-eight asymptomatic and sixty-two symptomatic shoulders were identified from a cohort of patients with unilateral shoulder pain related to rotator cuff disease. All shoulders underwent ultrasonographic evaluation of the rotator cuff and standardized radiographic evaluation. Humeral migration was measured by three observers using software-enhanced radiographic analysis. RESULTS There was no significant difference in rotator cuff tear size between the asymptomatic and symptomatic shoulders, although more tears involved the infraspinatus in the symptomatic group (p = 0.01). Proximal humeral migration was greater in the shoulders with a symptomatic tear than it was in those with an asymptomatic tear (p = 0.03). Tears that involved the infraspinatus resulted in more migration than did isolated supraspinatus tears in both the symptomatic (p = 0.01) and the asymptomatic shoulders (p = 0.03). When the symptomatic tears of > or =175 mm(2) were analyzed separately, the size of the tear was found to correlate strongly with humeral migration (p = 0.01). However, when the symptomatic tears that were <175 mm(2) were analyzed, neither tear size nor pain was found to have a significant relationship with migration. When the analysis was limited to full-thickness symptomatic tears of > or =175 mm(2), both pain (p = 0.002) and tear area (p = 0.0002) were found to have a significant effect on migration. Multivariate analysis showed that tear size (p = 0.01) was the strongest predictor of migration in symptomatic shoulders. CONCLUSIONS Proximal humeral migration correlates with rotator cuff tear size. Tears extending into the infraspinatus tendon are associated with greater humeral migration than is seen with isolated supraspinatus tears. Humeral migration resulting from symptomatic rotator cuff tears is greater than that resulting from asymptomatic tears. Additionally, there is a critical size for tendon tears resulting in humeral migration in painful shoulders. Although both pain and tear size influence glenohumeral kinematics in symptomatic shoulders, only tear size is an independent predictor of humeral migration.