Measuring peak expiratory flow rates (PEFR) several times a day can provide an objective assessment of functional changes relative to environmental or occupational exposures. The report describes the pattern of diurnal changes in PEFR in a reference population, and defines ranges of normal between-and within-day variability. An index of diurnal changes were defined as the ratio between maximal and minimal values, where the maximal value was restricted to PEFR measured at noon or in the evening (N,E) and the minimal value was restricted to the morning or at bedtime (M,B). A ratio greater than normal represented an exaggeration of the normal diurnal pattern in PEFR. Normal limits, based on the ninety-fifth percentile in the reference population, were larger for children (130%) than for adults 15 to 35 yr of age (117%) and those older than 35 yr of age (118%). The meaningfulness of excessive diurnal changes in PEFR was examined by relating this ratio (Max/Min), and a similar measure (the amplitude percent mean) to chronic respiratory symptoms and diseases in 939 adults and children who recorded PEFR values 2 to 4 times per day for as long as 14 days.