Peak expiratory flow rate (PEF) or forced expiratory volume in 1 s (FEV1) monitoring is an important clinical tool to assess the degree of 24-h variation (circadian) in lung function, which correlates with the degree of airway hyperresponsiveness. Ideally, PEF measurements should be taken frequently during the 24-h period to reflect the true variability. In practice, however, measurements are generally taken twice daily, usually upon arising and in the evening before bedtime. The effect of reducing the frequency of lung function measurement on the reported 24-h time average (mesor) and the circadian variability were evaluated. Four measurements taken 8 h apart and two measurements derived upon arising and the other, either 12 h later or before retiring, were compared with the full 24-h lung function profile, which included thirteen 2-hourly measurements. Based on the data from a 24-h lung function study in 20 adult patients with asthma, the corresponding values were separately compared at baseline and under two theophylline treatments, one being administered twice daily and the other once daily. The results are as follows: Whereas the 24-h time average (mesor) is hardly effected by the reduced measurements schemes, variability was grossly underestimated. Only 60 to 80% of the actual variability is picked up in the case of the four 8-hourly measurements, and only 20 to 45% in the case of the two measurements. The loss of accuracy in assessing 24-h airflow variability has potentially important implications on clinical practice and the investigation of therapeutics in asthma.(ABSTRACT TRUNCATED AT 250 WORDS)