The validity of peak expiratory flow (PEF) recordings and the sensitivity of PEF variability indices in asthma and asthma-like conditions were examined in an adolescent population. Recordings from 245 randomly selected subjects and from an additional 181 subjects reporting or considered at risk for developing asthma were analyzed. Subjects recorded PEF twice daily for 2 wk using Mini-Wright meters, completed a symptoms questionnaire, and were tested for airway responsiveness to methacholine. The first three recording days showed significant accumulation of lowest PEF values recorded (41%) and were excluded from further analysis. Among nine PEF variability indices, the Two-lowest%mean (the mean of the two lowest PEF values as a percentage of the period mean) had the best sensitivity for physician-diagnosed asthma (28%). The sensitivity of the methacholine dose-response slope (DRS) was 69%. Combining these indices, a sensitivity of 77% for diagnosed asthma was obtained. Among subjects with asthma-like symptoms but no diagnosis of asthma, 14% had increased Two-lowest%mean and the DRS was increased in 17%, but only 3% were identified by both tests. In conclusion, inhomogeneity of PEF data could be corrected by disregarding the first three recording days. PEF variability indices identified some diagnosed asthmatics, and particularly some symptomatic "nonasthmatics," not identified by the DRS, suggesting that the combined use of these indices might be helpful.