Aims/hypothesis Some studies have revealed a relationship between Caesarean section (CS) and type 1 diabetes, while other studies have not. By using the Swedish paediatric quality register we investigated whether birth by CS is related to the risk of developing type 1 diabetes during childhood. Methods All children diagnosed with type 1 diabetes from 2000 to 2012 and included in the register (n= 9,376) were matched with four controls by year, day of birth, sex and county of birth from the Swedish Medical Birth Register. Results Overall, 13.5% of deliveries were by CS. By group, 14.7% of children who developed type 1 diabetes were delivered by CS compared with 13.3% of control children (p < 0.001). Mothers with diabetes more often gave birth by CS than mothers without diabetes (78.8% vs 12.7%, p < 0.001). In a logistic regression model adjusting for maternal age, maternal diabetes and BMI in early pregnancy, the OR for CS was 1.0. A child who developed type 1 diabetes and had a mother with type 1 diabetes at the time of delivery had the highest OR to have been born by CS. Children of mothers without diabetes, delivered by CS, had no increased risk of developing type 1 diabetes. Maternal diabetes was the strongest predictor of childhood diabetes (OR 3.4), especially if the mother had type 1 diabetes (OR 7.54). Conclusions/interpretation CS had no influence on the risk of type 1 diabetes during childhood or adolescence. However, maternal diabetes itself strongly increased the risk of offspring developing type 1 diabetes.