The aim of our study was to investigate the association between rapid weight gain in the first 3 months of life and the prevalence of wheeze in the first years of life and lung function at 5 yrs of age. The infants selected were participating in an ongoing birth cohort. Information on growth and respiratory symptoms was collected during the first year of life, and on primary care consultations during total follow-up. Forced expiratory volume in 1 s (FEV(1)) and forced expiratory flow at 25-75% of forced vital capacity (FEF(25-75%)) were measured at 5 yrs of age. Information on growth and respiratory symptoms was obtained for 1,431 infants, out of whom 235 children had already had 5 yrs of follow-up. Every one-point z-score increase in weight gain resulted in a 37% increase in days with wheeze (incidence rate ratio 1.37, 95% CI 1.27-1.47; p<0.001) and in associated consultations by 16% (incidence rate ratio 1.16, 95% CI 1.01-1.34; p=0.04). Children with rapid weight gain reported significantly more physician-diagnosed asthma. FEV(1) and FEF(25-75%) were reduced by 34 mL (adjusted regression coefficient -0.034, 95% CI -0.056- -0.013; p=0.002) and 82 mL (adjusted regression coefficient -0.082, 95% CI -0.140- -0.024; p=0.006) per every one-point z-score increase in weight gain, respectively. These associations were independent of birthweight. Rapid early weight gain is a risk factor for clinically relevant wheezing illnesses in the first years of life and lower lung function in childhood.