Measuring peak expiratory flow (PEF) variation has been suggested as a indicator of asthma disease severity and also of nonspecific bronchial hyperreactivity. To test these assumptions, we examined the relationships between PEF variation, methacholine reactivity, symptom scores, and medication requirements in 74 children with tightly controlled allergic asthma. The level of mean diurnal variation (MDV) for the group was 7.1%, which is generally regarded as normal. We found statistically significant correlations between MDV and both methacholine reactivity (r = 0.43, p = 0.0001) and symptom scores (r = 0.28, p = 0.016). These asthma variables were analyzed longitudinally in 33 children who were followed up at 6-month intervals for at least 36 months. Visit-to-visit changes in MDV were generally not reflective of changes in other variables. However, group levels of MDV gradually decreased over time, especially in children with initial MDV of more than 8%. This reduction in group MDV coincided with similar reductions in group medication requirements and methacholine reactivity. We conclude that children with moderately severe asthma that is tightly controlled may have normal levels of PEF variation. The correlation between PEF variation and other asthma variables is statistically significant but too weak to be useful in the treatment of individual patients. In contrast, measurement of MDV may be a useful indicator of disease severity in group studies of asthma.