Diurnal variability of peak expiratory flow rates (PEFRs) was assessed in 1,237 children. The PEFR was measured twice daily over a 1-week period. As an index of variability, the log of a week's mean of daily amplitude was calculated. Linear regression analyses revealed a significant positive association between maternal smoking and the variability of PEFR for nonasthmatic children. For these children, exposure to maternal smoking was associated with a 13.7 percent increase (confidence interval [CI], 3.8 to 24.7 percent) in PEFR variability. For asthmatic children an effect was found for nonatopic (54.7 percent increase; CI, 5.5 to 226.8 percent) but not for atopic children (-8.5 percent change; CI, -41.2 to 42.3 percent). In the latter group, there was evidence that mothers changed their smoking habits subsequent to the development of disease in their children. We conclude that exposure to maternal smoking can increase the variability of PEFR and thus might contribute to the development of asthma.