INTRODUCTION: The risk of cardiovascular disease is increased in individuals with type 1 diabetes, despite good glycemic control. This study aims to evaluate early signs of atherosclerosis and predisposing factors in individuals with childhood-onset type 1 diabetes compared with healthy controls. RESEARCH DESIGN AND METHODS: The Atherosclerosis and Childhood Diabetes study is a prospective population-based cohort study with follow-up every fifth year. The cohort consists of 329 subjects with type 1 diabetes and 173 controls. Carotid intima-media thickness (cIMT) was measured at baseline and 5 and 10 years of follow-up. Data from the Norwegian Childhood Diabetes Registry were used in assessment of traditional risk factors. RESULTS: Mean cIMT in young women with type 1 diabetes increased significantly over a 10-year period compared with healthy controls (∆0.019 mm (0.001-0.035), p=0.035). At the 10-year follow-up the group with type 1 diabetes had a mean age of 24.2±2.9 years (13.7±2.8 years at baseline), diabetes duration of 15.6±3.4 years (5.4±3.3 years at baseline) and HbA1c of 8.2±3.6% (66±16 mmol/mol) (8.4±3.4% (68±13 mmol/mol) at baseline). Women with type 1 diabetes had significantly higher mean weight, body mass index, waist circumference, diastolic blood pressure (DBP), serum low-density lipoprotein (LDL)-cholesterol and apolipoprotein B, while men with type 1 diabetes had significantly higher mean DBP and urinary albumin-creatinine ratio compared with the control group. Mean cIMT change over time was not associated with long-term HbA1c or LDL-cholesterol burden in childhood and adolescence. CONCLUSION: Young women with childhood-onset type 1 diabetes of relatively short diabetes duration had a higher mean cIMT over a 10-year period compared with their healthy female controls, with values similar to males.