OBJECTIVE: To validate the polymyalgia rheumatica-activity score (PMR-AS). METHODS: Prospective cohort study at the University Hospitals Leuven (Belgium) between July 2022 and December 2023. We created a new alternative PMR-AS in which ability to elevate the upper limbs (EUL) was replaced by visual analogue scale (VAS) of functionality and both the severity and the duration of stiffness were evaluated. The discriminatory capacity for detecting active disease of the PMR-AS, the change in PMR-AS and several alternative scores were assessed using area under the receiver operating characteristic curves (AUC) and compared using Delong's tests. GEE-logistic prediction models were used to correct for repeated measures. RESULTS: We included 133 PMR patients (419 visits). PMR-AS had a good discriminatory power with an AUC of 0.938 and an optimal cut-off point of 11.0 with corresponding sensitivity of 87 % and specificity of 87 %. The change in PMR-AS (AUC 0.862), clinical-PMR-AS (AUC 0.928) and imputed-PMR-AS (AUC 0.928) significantly performed worse in detecting active disease (all p≤0.010). The ESR-PMR-AS also tended to have a lower discriminatory capacity (AUC 0.932, p=0.050). Our alternative PMR-AS had a higher AUC (AUC 0.948, p=0.010) with an optimal cut-off point of 13.5 and corresponding sensitivity of 87 % and specificity of 88 %. CONCLUSION: PMR-AS had a good discriminatory capacity for detecting active disease, but the alternative PMR-AS performed slightly better. Alternative activity scores which do not require the use of CRP performed slightly worse but can be used in case of treatment with glucocorticoid-sparing agents affecting the CRP level.