BACKGROUND: Many clinical and radiographic studies suggest that patient age and sex have an influence on rotator cuff (RC) repair outcomes. However, these findings result from retrospective statistical analyses and cannot provide a causal answer. PURPOSE: To analyze whether age and sex influence the biological potential at the time of RC repair or midterm clinical and radiographic outcomes. Also assessed was the effect of the biological potential on intraindividual clinical/radiographic results. STUDY DESIGN: Cohort study; Level of evidence, 2. METHODS: A total of 40 patients underwent arthroscopic RC repair. At the time of surgery (t = 0), supraspinatus tendon biopsy specimens were obtained, cultivated, and assessed for their biological potential, particularly (1) cell growth and (2) collagen type I production. After a follow-up at 24 months (t = 1), all patients were assessed by clinical scores (Constant score, subjective shoulder value, American Shoulder and Elbow Surgeons [ASES] score, and Western Ontario Rotator Cuff Index [WORC] score) and underwent magnetic resonance imaging to determine RC integrity. The data were examined for age- and sex-related differences and to identify the correlation between biological potential (t = 0) and clinical/radiographic outcome (t = 1). RESULTS: The follow-up rate for the imaging and clinical evaluation was 100%. Age, but not sex, influenced the biological tendon cell parameters at t = 0. However, there was no effect of age or sex on the clinical and radiographic results at t = 1. Furthermore, no correlation was observed between the initial biological parameters and later clinical outcomes or radiographic RC integrity. Finally, there was no significant difference between intact and nonhealed repairs in terms of the respective clinical scores. CONCLUSION: Age, but not sex, was found to have a negative effect on RC tendon cell biology. However, neither sex nor, in particular, a higher age influenced repair outcomes after 24 months.