Purpose: We assessed hypertrophy of preserved long head of the biceps tendon (LHBT) and vascularity in the bicipital groove after arthroscopic rotator cuff repair in 55-year-old patients and compared postoperative pain between shoulders with or without vascularity in the bicipital groove. Methods: Patients who underwent arthroscopic rotator cuff repair between 2015 and 2017 were reviewed. Inclusion criteria were arthroscopic rotator cuff repair and 55 years old. Exclusion criteria were a history of contralateral rotator cuff repair, revision surgery, partial repair or superior capsular reconstruction, shoulder dislocation or fracture, torn LHBT at surgery, LHBT tenodesis, retears, <1-year follow-up, and incomplete follow-up data. Cross-sectional area (CSA) of the LHBT and vascularity in the bicipital groove were examined preoperatively and 1 year after surgery using ultrasonography. Shoulder pain at postoperative 1 year was assessed using the pain subscore of the University of California at Los Angeles scale. The data were compared between shoulders with negative and positive vascularity. Results: Fifty-seven shoulders were included in this study. There was no side-to-side difference in preoperative CSA. No difference was found between preoperative and postoperative CSA in the affected shoulders. Postoperative vascularity was identified in 28 (49%) shoulders. Mean pain score was significantly higher in the negative vascularity group than the positive vascularity group (9 and 8, respectively; P ¼ .002). Conclusions: The preserved LHBT did not show hypertrophy 1 year after arthroscopic repair of medium-sized or smaller posterosuperior rotator cuff tear in 55-year-old patients. However, 49% of the shoulders postoperatively demonstrated lower-grade vascularity in the bicipital groove. Healthy LHBT can be preserved in 55-year-old patients with posterosuperior medium-sized or smaller rotator cuff tears. Level of Evidence: III, retrospective comparative prognostic trial