Recently, an unusual shape of the efficiency curve for a hydrocyclone classifier is reported by Bourgeois and Majumder [Powder Technology 237 (2013) 367-3751. They report actual recovery of near zero sized particles to underflow as zero. Such perfect separation of the finest particles is in total disagreement with the knowledge base and experience accumulated over many years of hydrocyclone practice. However, the authors assert that the efficiency curves reported by them are determined from a carefully generated experimental data followed by a rigorous statistical analysis. It is likely that their anomalous observations are due to systematic errors in size analyses. (C) 2014 Elsevier B.V. All rights reserved.