The objective of this study was to investigate the relationship between body mass index (BMI) and the incidence and progression of radiological knee as well as of radiological hip osteoarthritis. 3585 people aged 55 years were selected from the Rotterdam Study. The results showed that a high BMI (>27 kg/m2) at baseline was associated with incident knee osteoarthritis, but not with incident hip osteoarthritis. A high BMI was also associated with progression of knee osteoarthritis. For the hip, a significant association between progression of osteoarthritis and BMI was not found. The authors conclude that on the basis of these results, we conclude that BMI is associated with the incidence and progression of knee osteoarthritis. Furthermore, it seems that BMI is not associated with the incidence and progression of hip osteoarthritis.