BACKGROUND : The musculoskeletal tumour society score (MSTS) is a well-known questionnaire for measuring functional outcome in patients with neoplasms in the extremities. Standardized guidelines for cross-cultural translation and validation ensure the equivalence of content between the original and translated versions. The translation and validation provide the possibility to compare different sarcoma populations on an international level. This study is based on the hypothesis that the Danish MSTS questionnaire is a valid tool for measuring the end result after surgery for neoplasms in the extremities. OBJECTIVE : To validate the Danish version of the upper and lower extremity version of the MSTS. METHODS : The translation of the MSTS was conducted in accordance with international guidelines. Patients operated for sarcomas and aggressive benign tumors were invited to participate in the study. The psychometric properties of the Danish version of the MSTS were tested in terms of validity and reliability and for the risk of floor or ceiling effect. Spearman's rank coefficient was used to test the validity by comparing with the Toronto Extremity Salvage Score (TESS). The Intraclass Correlation Coefficient (ICC) was used to evaluate inter-rater reliability. Cronbach's alpha was used to test for internal consistency. Spearman's rank coefficient was used to compare the MSTS lower extremity version with the objective test, Timed Up and Go (TUG). RESULTS : The upper extremity version demonstrated an ICC of 0.95 in the inter-rater reliability test. The lower extremity version had an ICC of 0.88 in the inter-rater reliability test, respectively. Both MSTS versions showed a ceiling effect. The validity of the MSTS was measured by Spearman's rank correlation coefficient by comparing the MSTS with the TESS and found it to be of 0.80 (P < 0.01) and 0.83 (P < 0.01) for the upper extremity and lower extremity version, respectively. A Spearman's rank correlation coefficient of - 0.26 (P < 0.01) was found between the TUG and the MSTS questionnaire. A Spearman's rank correlation coefficient of - 0.38 (P < 0.01) was found between the TUG and the lower extremity version of the TESS questionnaire. CONCLUSIONS : The Danish version of the MSTS questionnaires were found to have good reliability and validity, however a substantial ceiling effect was identified.