The aim of this study was to evaluate the long-term functional outcome of full thickness rotator cuff tears treated by open repair and acromioplasty and to determine various factors affecting the outcome. This is a prospective clinical study on 42 patients who underwent full thickness rotator-cuff repair by a single surgeon between 2000 and 2003. The mean follow-up was 26 months. In patients with massive rotator cuff tear (n = 15), increase in the mean postoperative Constant score was significantly less compared to patients with small and moderate tears (p < 0.01). In patients older than sixty years, the improvement in postoperative Constant score was significantly less compared to the rest of the patients (p < 0.001). However, the postoperative Constant score significantly improved from the preoperative score in all patient groups (p < 0.0001). There was a significant negative correlation with size of cuff tear, age of the patient and Constant-Murley score (p < 0.05). Body mass index, smoking, gender, and the duration of symptoms did not have a significant effect on either Constant-Murley score or visual analogue score. We conclude that older patients and those with massive rotator cuff tear could benefit from surgical intervention, although not as much as younger patients and those with small/moderate size cuff tears.