Obesity is associated with high bone mineral density (BMD), but whether obesity-related higher bone mass increases bone strength and thereby protect against fractures is uncertain. We estimated effects of obesity on bone microarchitecture and estimated strength in 36 patients (12 males and 24 females, age 25-56 years and BMI 33.2-57.6 kg/m(2)) matched with healthy controls (age 25-54 years and BMI 19.5-24.8 kg/m(2)) in regard to gender, menopausal status, age (±6 years) and height (±6 cm) using high resolution peripheral quantitative computed tomography and dual energy X-ray absorptiometry. In radius, total bone area and trabecular area were significantly higher in obese patients (both p < 0.04). In tibia, cortical area was larger in obese patients (p < 0.001) compared with controls. Total BMD was higher in tibia (p = 0.03) but not in radius. Trabecular integrity was strengthened in obese patients compared with controls in radius and tibia with higher trabecular number (p = 0.002 and p < 0.001) and lower trabecular spacing (p = 0.01 and p < 0.001). Finite element analysis estimated failure load (FL) was higher in tibia (p < 0.001), but not in radius in obese patients. FL was significantly lower per kg body weight in radius and tibia in obese patients compared with controls (p = 0.007 and p < 0.001). Furthermore, the ratios of FLs between groups were comparable in both sites. These findings suggest that mechanical loading is not the primary mediator of the effects of obesity on estimated FL, and suggest that bone strength adaptations in morbid obesity may be inadequate with respect to the increased mechanical demands.