When tested according to the European standard, the performance of breathing system filters is determined at a flow of 15 or 30 l.min-1 for filters intended for use with paediatric or adult patients, respectively. However, higher flows of gas may pass through a filter in some circumstances. The penetration of sodium chloride particles through seven different breathing system filters (three pleated hydrophobic and four electrostatic) was measured at five different flows ranging from 15 to 75 l.min-1. Penetration varied from 0.004% to 24.4% for the various filters at the different flows. Penetration increased by between 2 and 40 times for the different filters as the flow increased by a factor of five but this did not markedly alter the rank order of the filters in terms of performance. Testing to the standard provides a useful indication of relative performance at any flow.