Background: Research suggests an influence of micronutrients on childhood asthma. So far, evidence mainly originates from cross-sectional studies using nutrient intake data, which is not an accurate measure of nutrient status. This study aimed to investigate the cross-sectional and prospective associations between serum concentrations of magnesium, vitamin D, selenium, and zinc and prevalence of (severe) asthma, atopy, and bronchial hyperresponsiveness (BHR) in childhood. Methods: In the Prevention and Incidence of Asthma and Mite Allergy birth cohort study, serum nutrient concentrations were available for a 4-yr-old subgroup (n = 372) and for a different 8-yr-old subgroup (n = 328). Yearly questionnaires inquired about asthma prevalence until 8 yr of age. Allergic sensitization was measured at 4 and 8 yr of age; BHR was measured at 8 yr of age. Data were analyzed with logistic regression and generalized estimating equations models. Results: There was a consistent (non-significant) inverse association between serum magnesium concentrations and asthma prevalence. Serum vitamin D concentrations measured at age 4 were inversely associated with asthma at ages 4–8 [e.g., cross-sectional association between vitamin D tertile 3 vs. 1 and severe asthma: odds ratio (OR): 0.49, 95% confidence interval (CI): 0.25–0.95], whereas vitamin D measured at age 8 was positively associated with asthma at age 8 (e.g., cross-sectional association between vitamin D tertile 3 vs. 1 and severe asthma: OR: 2.14, 95% CI: 0.67–6.82). Conclusions: Our study contributes to the evidence that children with higher serum magnesium concentrations are less likely to have asthma. The associations between serum vitamin D concentrations and asthma were age-dependent
