OBJECTIVE: To examine the association between body weight measures across the lifecourse and the risk of adult-onset diabetes. METHODS: We analysed data from the Glasgow Alumni Cohort and the British Women's Heart and Health Study (BWHHS). The former included 5,571 men and women who had height and weight measured at university, and reported birthweight, mid- and later-life weight in a postal questionnaire. The BWHHS analysis included 4,280 women who had height and weight measured in later adulthood and recalled their birthweight and early adult height and weight. Adult-onset diabetes was defined as doctor-diagnosed disease after age 30, either self-reported or abstracted from medical records. RESULTS: Thirty nine women and 209 men (Glasgow Alumni study) and 314 women (BWHHS) had diabetes. Those with diabetes had lower mean birthweight than those without, although the differences were small. Individuals with diabetes were also shorter and heavier at all ages than those without diabetes. Being overweight during at least one time period in adult life was associated with an increased risk of diabetes, compared to those who were never overweight. While there was no age at which being overweight was particularly detrimental, the risk associated with being overweight was cumulative across the lifecourse. CONCLUSIONS: Being overweight at any point during life is associated with an increased risk of adult-onset diabetes. The cumulative nature of this association reinforces the need to prevent the development of excess weight at an early age to reduce diabetes prevalence in coming decades.