Peak expiratory flow (PEF) variation was measured in 18 healthy children (Group I) aged 9.25 to 13.3 years (mean, 11 years) and in 17 asymptomatic asthmatic children (Group II) aged 8.75 to 14.25 years (mean, 11.9 years) receiving no bronchodilators. PEF was measured four times a day for a period of 10-14 days. There were no differences between Group I and Group II regarding age, height, weight, sex, duration of the study, and overall mean PEF values. Mean daily PEF variation amplitude (difference between the highest and the lowest daily values as a percent of the mean) was significantly higher in Group II than in Group I subjects (15.2 vs. 9.9%), and so was cosinor amplitude (9.6 vs. 5.9%). Significant circadian rhythm in PEF was detected by cosinor analysis in nine subjects of Group I and in eight subjects of Group II. Mean PEF bathyphase calculated by cosinor analysis (trough time in a 24 h scale) was similar in the two groups (0503 h, in Group I and 0434 h in Group II). The results of this study show that the amplitude of circadian variation in PEF tends to be higher in asymptomatic asthmatic children than in normal children, but there is too much overlap to use PEF monitoring to identify an individual asymptomatic asthmatic child. Therefore, diurnal variation of PEF is not suitable for diagnosing mild asthmatics.