60 patients with rotator cuff tendinopathy (subacromial impingement or partial thickness tear) were randomized to receive arthroscopic acromioplasty with or without an additional injection of platelet-rich plasma (PRP). The purpose of this randomized controlled trial was to assess whether PRP injection affected clinical outcomes and histological tissue characteristics. The results displayed no significant differences between the two groups in terms of Oxford Shoulder Scores, VAS pain, satisfaction, or quality of life at 2 years postoperatively. The patients treated with PRP displayed decreased numbers of blood vessels and tendon cellularity, with increased expression of p53 (apoptosis marker), in the tendon biopsy. Bonar scoring displayed no significant changes in tissue structure between groups.