Numerous studies have reported the so-called fish-hook effect on granulometric efficiency curves of hydrocyclones. This effect has been reported since laser diffraction particle size analysers started to be used. In this work a 15-mm-diameter Bradley hydrocyclone has been used to separate a suspension of calcium carbonate 1,2% in volume to analyse the fish-hook effect. The Malvern Mastersizer 2000 particle size analyser has been used and both Fraunhofer's and Mie's light scattering theories have been assessed. It has been concluded that the fish-hook effect is a result of using Fraunhofer's correlation to determine the size of particles which are knowingly out of its range of validity (diameters smaller than 50 mu m) or using Mie's correlation with inappropriate particles' imaginary refraction indexes (IRI). A method to determine the appropriate IRI in the Malvern Mastersizer 2000 is also presented. The fish-hook effect is therefore a misuse of laser diffraction particle size analysers. (C) 2020 Elsevier B.V. All rights reserved.