Diabet. Med. 29, e369-e376 (2012) ABSTRACT: Aims To assess physical activity and fitness levels of young people with Type 1 diabetes compared with siblings without diabetes, and to investigate the association between physical activity, physical fitness and glycaemic control (HbA(1c) ) in those young people with diabetes. Methods The study consisted of 97 young people aged 8 to 16 years (62% male) from a Paediatric Diabetes Service in South West England. Sixty participants (67% male) had Type 1 diabetes and 37 participants (54% male) were siblings without diabetes (control group). We measured weight, height and waist circumference, calculated BMI and waist-height ratio and recorded pubertal status, blood pressure and current insulin regimen information. We assessed physical activity by accelerometry, from which we calculated light and moderate-to-vigorous intensity activity. We measured physical fitness by multistage sub-maximal bicycle ergometer test. We obtained HbA(1c) by venipuncture. Results There were no differences between the young people with diabetes and siblings without diabetes in body composition, blood pressure, physical activity and fitness. Moderate-to-vigorous physical activity was associated with better glycaemic control, accounting for 30-37% (R(2) = 0.295-0.374) of the variance for HbA(1c) . Physical fitness was not associated with HbA(1c.) Conclusions Moderate-to-vigorous physical activity was associated with better glycaemic control while fitness was not. Findings suggest that developing strategies to increased moderate-to-vigorous physical activity may prove an effective method of improving glycaemic control in young people with diabetes.