The aim of this study was to validate clinical vignettes as a measure of physiotherapists’ activity and work recommendations given to patients with non-specific low back pain. This was a wvlidation study comparing two methods for measuring aspects of health providers’ clinical management: Clinical vignettes and unannounced visits of standardized patients (the gold standard). Physiotherapists (N = 59) who consented to see unannounced standardized patients in their clinical practice. Clinical vignettes were used to initially measure physiotherapists’ self-reported activity and work recommendations. Subsequently, actors performing as standardized patients visited physiotherapists in their clinical practice and rated the advice given by the physiotherapist regarding activity and work. A total of 23 standardized patients were randomly scheduled to physiotherapists. Physiotherapists were blinded towards the standardized patients. To test whether standardized patients were detected, physiotherapists reported if they suspected that they had treated an actor. The 23 standardized patients visited 22 different physiotherapists. Physiotherapists detected 12 out of 23 unannounced standardized patients (detection rate: 52%). The estimated agreement between the two measures was poor, for both activity and work recommendations (weighted kappa coefficients: 0.29 resp. -0.21).