Preterm birth (gestational age <37 weeks) has been associated with insulin resistance early in life. However, no large population-based studies have examined risks of type 1 and type 2 diabetes and potential sex-specific differences from childhood into adulthood. Clinicians will increasingly encounter adults who were born prematurely and will need to understand their long-term risks. We hypothesised that preterm birth is associated with increased risks of type 1 and type 2 diabetes into adulthood.A national cohort study was conducted of all 4,193,069 singletons born in Sweden during 1973-2014, who were followed up for type 1 and type 2 diabetes identified from nationwide diagnoses and pharmacy data to the end of 2015 (maximum age 43 years; median age at the end of follow-up 22.5 years). Cox regression was used to adjust for potential confounders, and co-sibling analyses assessed the influence of shared familial (genetic and/or environmental) factors.In 92.3 million person-years of follow-up, 27,512 (0.7%) and 5525 (0.1%) people were identified with type 1 and type 2 diabetes, respectively. Gestational age at birth was inversely associated with both type 1 and type 2 diabetes risk. Adjusted HRs for type 1 and type 2 diabetes at age <18 years associated with preterm birth were 1.21 (95% CI, 1.14, 1.28) and 1.26 (95% CI, 1.01, 1.58), respectively, and at age 18-43 years were 1.24 (95% CI, 1.13, 1.37) and 1.49 (95% CI, 1.31, 1.68), respectively, compared with full-term birth. The associations between preterm birth and type 2 (but not type 1) diabetes were stronger among females (e.g. at age 18-43 years, females: adjusted HR, 1.75; 95% CI, 1.47, 2.09; males: 1.28; 95% CI, 1.08, 1.53; p < 0.01 for additive and multiplicative interaction). These associations were only partially explained by shared genetic or environmental factors in families.In this large national cohort, preterm birth was associated with increased risk of type 1 and type 2 diabetes from childhood into early to mid-adulthood. Preterm-born children and adults may need early preventive evaluation and long-term monitoring for diabetes.