OBJECTIVES: Patient-reported outcomes (PRO) assessing health-related quality of life (HRQoL) are important outcome measures, especially in Legg-Calvé-Perthes disease (LCPD) where symptoms (pain and limping), activity restrictions, and treatments vary depending on the stage of the disease. The purpose of this study was to investigate the validity of the Patient-reported Outcomes Measurement Information System (PROMIS) for measuring HRQoL of patients with LCPD in various stages of the disease. METHODS: This is a multicenter validity study. Patients with LCPD between 4 and 18 years old were included and classified into modified Waldenström stages of disease: Early (1 or 2A), Late (2B or 3), or Healed (4). Seven PROMIS domains were collected, including Pain Interference, Fatigue, Mobility, Depression, Anger, Anxiety, and Peer Relationships. Convergent, discriminant, and known group validity was determined. RESULTS: A total of 190 patients were included (mean age: 10.4±3.1 y). All 7 domains showed the worst scores in patients in the Early stage (known group validity). Within each domain, all domains positively correlated to each other (convergent validity). Patients who reported more anxiety, depression, and anger were associated with decreased mobility and increased fatigue and pain. Peer relationships had no to weak associations with other domains (discriminant validity). CONCLUSIONS: PROMIS has construct validity in measuring the HRQoL of patients in different stages of LCPD, suggesting that PROMIS has potential to serve as a patient-reported outcome tool for this population. LEVEL OF EVIDENCE: Diagnostic level III study.