Diurnal variability in peak expiratory flow (PEF) has been an accepted
clinical method in the management of asthma and the evaluation of occ
upational asthma. In this paper, the basis for this usage together wit
h other clinical and epidemiological applications is discussed, The me
asured characteristics of PEF diurnal variability are described in ast
hmatics and asymptomatic subjects, showing the greater variability in
asthmatics, and the ''morning dip'' related to circadian rhythm, PEF m
easured by pneumotachograph and the mini-Wright meter are shown to be
in good agreement, but PEF measured by the latter and other small PEF
meters is different in terms of absolute values, and both intraindivid
ual variability within test sets and diurnally, The use of PEF meters,
and the daily diaries in which the subject or patient records PEF and
related factors are described.