To investigate the effects of dietary magnesium, potassium, and sodium on children's lung function, the authors examined cross-sectional dietary data and pulmonary function tests from 2,566 children aged 11-19 years who attended schools in 12 southern California communities during 1998-1999. During school visits, each child completed a health update questionnaire, a validated food frequency questionnaire, and spirometric lung function testing. Low magnesium and potassium intakes were associated with lower lung function. Girls with low magnesium intake had lower forced expiratory flow at 75% of the forced vital capacity (FEF(75)) (-8.3%, 95% confidence interval: -14.8, -1.4) than did girls with higher intake; reductions were larger in girls with asthma (forced expiratory flow between 25% and 75% of the forced vital capacity (FEF(25-75)) (-16.2%, 95% confidence interval: -22.7, -9.1) and FEF(75) (-24.9%, 95% confidence interval: -32.8, -16.1)) than in girls without asthma (FEF(25-75) (-2.0%, 95% confidence interval: -7.4, 3.8) and FEF(75) (-4.1%, 95% confidence interval: -11.3, 3.7)). Boys with low magnesium intake showed deficits in forced vital capacity (-2.8%, 95% confidence interval: -5.4, -0.2) compared with boys with higher intake. The effects of low magnesium intake did not vary substantially in boys with and without asthma. Among girls, low potassium intake was also associated with deficits in forced expiratory volume in 1 second (-2.7%, 95% confidence interval: -5.2, -0.1) and forced vital capacity (-2.4%, 95% confidence interval: -4.7, -0.1). In summary, low magnesium and potassium intakes were associated with lower lung volumes and flows.