Diurnal variability in peak expiratory flow (PEF) was studied for three days in 152 healthy volunteers aged 20-40 years, and expressed as amplitude percent mean (A%M) and standard deviation percent mean (SD%M). The commonest pattern (74.6%) observed was that of a nadir at waking in morning, a progressive rise upto late afternoon and a plateau or a small decrease at bedtime. Mean diurnal variation in PEF on the third day was 7.23 +/- 3.48 (A%M) and 2.98 +/- 1.31 (SD%M). Both the diurnal variation and the calculated upper limits of normality were found to be lower than those reported previously for Western subjects. This knowledge of diurnal variation is important for any meaningful interpretation of PEF recordings used to monitor patients with asthma.