The replacement of lung function equipment may not result in changing values: transitions should be seamless. We compared the equipment of two manufacturers (ZAN and Jaeger) to estimate the differences in spirometry, bodyplethysmography and diffusion capacity derived parameters elicited by equipment differences, and also compared calibration systems. From Jaeger (1) a Masterlab bodyplethysmograph, (2) a Pneumoscreen pneumotachograph, (3) a Masterscreen CS-FRC and (4) a Masterlab CompactTransfer (the latter two measure the diffusion capacity) were compared, and from ZAN a bodyplethysmograph system encompassing spirometry, flow-volume curve and diffusion capacity measurements. In vitro (a 2 litre calibration syringe at three flows, a fixed flow generator (0.5 l s(-1), 1.67 l s(-1) and 5 l s(-1)) and a wave form generator) studies and an in vivo cross-over study (N = 59) were used to compare the pneumotachographs. Other parameters were compared only via the in vivo study. The calibration syringe study showed no differences between the pneumotachographs: all volumes in all systems were within 3% of the reference. The fixed flow generator showed no flow differences either: again within 3% of the reference. The wave form generator showed PEF, MEF(75/50/25) differences up to 10-15% from the reference, but none in FEV(1) and FVC. The in vivo study found similar system differences in PEF, MEF(75/50/25): up to 29%. FEV(1), VC and FVC differed by small amounts (in all cases <3% between systems). R(0.5), TLC, V(A) and RV differed by small amounts between the systems (in all cases <3%), but T(L,CO) up to 9%. This study showed that between and within manufacturers, significant differences can exist between pneumotachographs and that using calibration syringes or fixed flow generators these frequently go unnoticed. These approaches insufficiently test the dynamic features of pneumotachographs. For other parameters, in vivo calibration is the only option and accuracy is only achieved with rather large samples, so more research is needed into suitable in vitro systems.