This study was designed to estimate the validity of an 11-point verbal numerical rating scale (VNRS) and a 100 Unit (U) plasticized visual analogue scale (VASp) using a 100mm paper visual analogue scale (VAS) as a gold standard, to recommend the best method of reporting the intensity of acute pain in an emergency department (ED). A convenience sample of 1176 patients with acute pain were recruited in the ED of a teaching hospital. Patients >18 years and able to use the different scales were included. Scales were presented randomly. Results were converted to a 0-100 U scale and validity was quantified using the Bland-Altman method and the intra-class correlation (ICC). The limits of acceptability were previously set for the limits of agreement at +/-20 U, with a constant bias. The Bland-Altman method revealed a small bias of -4 U for the VNRS and +1 U for VASp. However, the bias of the VNRS varied with the intensity of pain from -10 to +1 U. The limits of agreement between the VNRS&VAS and the VASp&VAS were -25; +17 U and -17; +18 U, respectively. The ICC was excellent between the VNRS&VAS (0.88) and the VASp&VAS (0.92). In conclusion, the VASp has a small bias, acceptable limits of agreement and an excellent intra-class correlation. It is probably a valid tool to estimate acute pain in the ED. However, the VNRS is less valid in that context because of its wide limits of agreement and variable bias (mainly in lower scores).