Abstract Background Hypertension, type 2 diabetes and other cardiovascular diseases, all risk factors for atrial fibrillation, are associated with birth weight. It remains unclear, however, whether risk of atrial fibrillation is also associated with birth weight. We investigated the associations of birth weight and preterm birth (i.e., born more than one month before term) with risk of atrial fibrillation (AF). Methods The study population comprised 29 551 men and 23 454 women who were free from AF at baseline. Information on birth weight, preterm birth, and risk factors for AF was obtained from a questionnaire. Incident AF cases were ascertained by linkage to the Swedish Inpatient Register. Results During 12 years of follow-up, AF developed in 2711 men and 1491 women. High birth weight (≥ 5000 g) was associated with an increased risk of AF after adjustment for age and other risk factors for AF, but the association did not persist after further adjustment for adult height. In men but not in women, low birth weight was associated with an increased risk of AF. Compared with men weighing 2500–3999 g at birth, the multivariable RR was 1.86 (95% CI, 1.15 to 3.00) for those weighing < 1500 g. This association was stronger in men who were born full-term (RR 2.53; 95% CI, 1.35 to 4.73). Conclusions Both high birth weight and low birth weight (in men), in particular in men born full-term, were associated with an increased risk of AF. The association with high birth weight appeared to be mediated through adult height.