Summary. Although body adiposity and disease severity in haemophilia have been found in cross-sectional studies to be negatively associated with joint mobility, it is not clear how these two factors affect the rate of joint mobility loss over time. Over a 10-year period, repeated measures of joint range of motion (ROM) were collected annually using universal goniometers on bilateral hip, knee, ankle, shoulder and elbow joints in 6131 young males with haemophilia A aged ≤20 years. Body mass index (BMI) was calculated using data on weight and height during follow up. The effect of body adiposity, adjusted for disease severity, on the rate of joint mobility loss over time was assessed using a longitudinal model. Compared with haemophilia males with normal BMI, those who were obese had lower ROM at initial visit and a faster rate of joint mobility loss in the lower limbs. Overweight subjects experienced similar loss in ROM, although to a lesser degree. A decline in ROM with age was also observed in upper limb joints but the rate was not significantly affected by body adiposity. Haemophilia severity, joint bleeding and the presence of an inhibitor were other significant contributors to joint mobility loss in both upper and lower limb joints. Excess body adiposity accelerates joint mobility loss in weight bearing joints particularly among those with severe haemophilia. Our findings suggest that body weight control and effective treatment of bleeds should be implemented together to achieve better joint ROM outcomes in males with haemophilia.