This paper describes a systematic study of the influence of the inlet clearance gap on the performance of a centrifugal fan. Overall fan performance in terms of volume flow rate, pressure rise, stall margin, and efficiency were measured over a range of values of the radial clearance between the impeller and the stationary inlet cone. These data have been correlated as functions of lumped clearance parameters. Additional data on velocity surveys in the impeller discharge are presented and discussed in relation to overall performance.