Electronic devices are now available to measure and store lung function parameters in the home. Before adopting a device for clinical or research use, it is important to validate it in the target patient group. The aim of this study was to assess a low-cost, portable, logging spirometer, the VM Plus (VM), against a standard laboratory Jaeger spirometer (JS) for use in children with respiratory disease. Seventy children with stable asthma or cystic fibrosis performed spirometry on the two devices, and results for peak expiratory flow (PEF) and forced expiratory volume in 1 sec (FEV(1)) were compared. Comparison was made both using the two devices separately (separate method) and with the devices connected in series (series method). Reproducibility of the VM measurements was also assessed. Correlation between measurements was close (R values: separate, PEF, 0.91; FEV(1), 0.94; series, PEF, 0.97, FEV(1), 0.99), but PEF readings on the VM Plus were substantially higher than with the JS (mean difference: separate, 54.8 L/min; series, 28.2 L/min). This reflects well-reported differences in PEF measurements between the Mini-Wright PEF meter, on which the VM Plus spirometer is based, and conventional spirometers. Limits of agreement (series method) were: PEF, -13.2 to +69.6 L/min; FEV(1), -0.03 to +0.19 L. Reproducibility of VM Plus measurements was acceptable: coefficient of variation for PEF was 4%; for FEV(1), 4.3%; coefficient of reproducibility for PEF, 39 L/min; for FEV(1), 0.26 L. The VM Plus provides reasonably accurate, reproducible measurements of PEF and FEV(1), but intrinsic bias, particularly in PEF measurement, needs to be taken into account. Its potential to document longitudinal changes in lung function in children with respiratory disease at home merits further study.