The Copenhagen Hip And Groin Outcome Score (HAGOS) was developed to assess disease-specific consequences in young to middle-aged, physically active hip and/or groin patients. The study aimed to determine validity and reliability of the Dutch version of the HAGOS (HAGOS-NL) for middle-aged patients with hip complaints.To assess validity, 117 participants completed five questionnaires: HAGOS-NL, international Hip Outcome Tool (iHOT-12NL), Hip disability and Osteoarthritis Outcome Score (HOOS), RAND-36 Health Survey and Tegner activity scale. Structural validity was determined by conducting confirmatory factor analysis. Construct validity was analyzed by formulating predefined hypotheses regarding relationships between the HAGOS-NL and subscales of the iHOT-12NL, HOOS, RAND-36 and Tegner activity scale. The HAGOS-NL was filled out again by 67 patients to explore test-retest reliability. Reliability was assessed in terms of Cronbach's alpha, Intraclass Correlation Coefficient (ICC), Standard Error of Measurement (SEM) and Minimal Detectable Change (MDC). The Bland and Altman method was used to explore absolute agreement.Factor analysis confirmed that the HAGOS-NL consists of six subscales. All hypotheses were confirmed, indicating good construct validity. Internal consistency was good, with Cronbach's alpha values ranging from 0.89 to 0.98. Test-retest reliability was considered good, with ICC values of 0.80 and higher. The SEM ranged from 6.6 to 12.3, and MDC at individual level from 18.3 to 34.1 and at group level from 2.3 to 4.4. Bland and Altman analyses showed no bias.The HAGOS-NL is a reliable and valid instrument for measuring pain, physical functioning and quality of life in middle-aged patients with hip complaints.