OBJECTIVE: The aim of this study was to examine this association between maternal weight during pregnancy and the incidence of hospitalisations for infectious diseases during early childhood. DESIGN: A population-based cohort study. SETTING: A national cohort was created by combining data from the Swedish Medical Birth Register, the National Inpatient Register, the Cause of Death Register, the Total Population Register and the Longitudinal integration database for health insurance and labour market studies. PATIENTS: 693 007 children born in Sweden between 1998 and 2006. MAIN OUTCOME MEASURES: Number of hospitalisations for infectious diseases during the first 5 years of life, overall and for categories of infectious diseases (lower respiratory, enteric, upper respiratory, genitourinary, perinatal, skin and soft tissue, neurological and eye, digestive tract, bloodstream and other infections). RESULTS: Overweight (body mass index (BMI) 25.0-29.9) and obesity (BMI≥30) during pregnancy were associated with a higher overall incidence of hospitalisations for infectious diseases, adjusted incidence rate ratio (IRR) 1.05 (95% CI 1.03 to 1.06) and adjusted IRR 1.18 (95% CI 1.16 to 1.21). Overweight and obesity during pregnancy were strongly associated with perinatal infections, adjusted IRR 1.34 (95% CI 1.25 to 1.44) and adjusted IRR 1.72 (95% CI 1.57 to 1.88). In contrast, we found no association between maternal weight during pregnancy and infections of skin and soft tissue, the nervous system, the digestive tract or the bloodstream. CONCLUSIONS: We observed an association between overweight and obesity during pregnancy, and hospitalisations for infectious diseases during early childhood.