INTRODUCTION: Shoulder pain is one of the most common complaints in orthopaedics. This study focusses on the relationship between shoulder function in subacromial impingement syndrome and imaging criteria in magnetic resonance imaging (MRI). MATERIALS AND METHODS: This prospective clinical trial included 69 patients treated for subacromial impingement syndrome. Shoulder function (Constant Score, range of abduction, abduction force) and pain were correlated with the following MRI parameters: tendinosis of the rotator cuff, "halo-sign" around the biceps tendon, subacromial distance, critical shoulder angle, size of subacromial osteophytic spurs and maximum width of subacromial and subdeltoid bursa. Statistical analyses included Pearson's and Spearman's coefficients of correlation, multiple regression analysis and Student's t-test. RESULTS: The Constant Score was correlated positively with the critical shoulder angle (r = 0.313; p = 0.009) and inversely with a "halo-sign" around the biceps tendon (rho = -0.384; p = 0.001). There was no significant correlation between spur size and shoulder function, but the size of the subacromial and subdeltoid bursae was positively correlated with the subacromial spur's size (subacromial bursa: coronal plane: r = 0.327; p = 0.006; sagittal view: r = 0.305; p = 0.011; subdeltoid bursa coronal view: r = 0.333 p = 0.005). The width of the subdeltoid bursa in coronal plane was positively correlated with shoulder pain (r = 0.248; p = 0.004) and negatively with the range of abduction (r = -0.270; p = 0.025), as well as the mean (r = -0.332; p = 0.005) and maximum (r = -0.334; p = 0.005) abduction force. CONCLUSIONS: Shoulder function and pain in subacromial impingement are best predicted by the width of the subdeltoid bursa measured in the coronal MRI plane as an indicator of bursitis as well as the presence of a "halo-sign" around the biceps tendon indicating glenohumeral joint effusion. Presence of a subacromial spur could lead to subacromial and subdeltoid bursitis, which impairs shoulder function. Shoulder function seems not to be compromised by the presence of a subacromial spur in absence of bursitis. This study was registered at the German Clinical Trials Register on 08 February 2013 (ID: DRKS00011548).