Low serum folate levels and the TT genotype of the MTHFR-C677T polymorphism were associated with increased prevalence of self-reported doctor-diagnosed asthma [odds ratio (OR) 1.37, 95% confidence interval (CI) 1.05-1.79 and OR 1.52; 95% CI 1.12-2.06, respectively] and attacks of shortness of breath (OR 1.43, 95% CI 1.14-1.79 and OR 1.47; 95% CI: 1.14-1.91, respectively).