The American Thoracic Society/European Respiratory Society Task Force underlined that the use of in-line filters during respiratory function tests ''is an area of controversy''. The aim of the present study was to measure the contamination occurring during forced expiration downstream from a screen pneumotachograph (SP) with and without an in-line filter (Pall PF30S). A total of 40 healthy subjects performed eight consecutive maximal expiratory manoeuvres into four sterile apparatuses (A1: no filter, no SP; A2: filter-only; A3: SP-only; A4: filter and SP) in random order. A blood agar plate was fixed downstream from the apparatus. Colonyforming units (CFUs) were counted after 24 h incubation at 37uC. Of the 40 plates obtained with each apparatus, 13 were sterile with A1 (range 0-679 CFUs), 25 with A2 (0-49 CFUs), 30 with A3 (0-35 CFUs) and 39 with A4 (one CFU in the only positive plate). A1 versus A2 and also A3 versus A4 gave different values for the CFU number, but A2 and A3 showed similar contamination levels.The authors conclude that: 1) the in-line filter does not perform better than a screen pneumotachograph; 2) it does not eliminate the need to decontaminate the pneumotachograph; and 3) equipment placed downstream from an in-line filter and a screen pneumotachograph is almost protected from contamination.