Current reference values for diurnal peak flow variation in healthy children (median 8.2%; 95th percentile 31%) are so high that considerable overlap exists with those of asthmatic children. These values have been obtained using written peak flow diaries, which are unreliable. The aim of the present study was to obtain reliable reference values for the variation in peak flow and forced expiratory volume in one second (FEV(1)) in healthy schoolchildren using home spirometry with electronic data storage. Healthy schoolchildren (n = 204; 100 males) aged 6-16 yrs measured their peak flow and FEV(1) twice daily for 2 weeks using an electronic home spirometer. The variation in peak flow and FEV(1) were calculated as a diurnal amplitude as a percentage of the day's mean. The mean peak flow variation was 6.2% (95th percentile 12.3%) and the mean FEV(1) variation was 5.7% (95th percentile 11.8%). Using home spirometry with electronic data storage, healthy schoolchildren show considerably less peak flow and forced expiratory volume in one second variation than previously reported on the basis of written peak flow diaries. Being the 95th percentiles of the distributions in healthy children, a peak flow variation of 12.3% and an forced expiratory volume in one second variation of 11.8% are suggested as cut-off values for disease when using home spirometry.