The aim of this study was to identify factors associated with fractures in patients with postmenopausal osteoporosis. The overall hypothesis was that trunk muscle strength, body sway and hypovitaminosis D would influence daily activities and the likelihood of falls and fractures. - In 237 women (mean age 62.9+/-7.4 years) osteoporosis was defined by a T-score at the femoral neck below -2.5 SD. Trunk muscle strength was determined using isokinetic dynamometry and body sway was measured according to Lord et al. Limitations in everyday life were assessed and the history of falls was documented. A fracture was defined as a vertebral height reduction of more than 20% or at least 4 mm. The assessment was carried out using the Spine Deformity Index (SDI) and was confirmed by an experienced radiologist. Pearson coefficients of correlation were calculated. - After correction for age, significant associations were found for body sway and 25-hydroxyvitamin D (p<0.001), body sway and falls (p<0.001), body sway and rib fractures (p<0.01), trunk muscle strength and limitations in everyday life (p<0.001), trunk muscle strength and SDI (p<0.001), trunk muscle strength and bone density (p<0.001), and bone density and 25-hydroxyvitamin D (p<0.001). No significant correlation was found for trunk muscle strength and 25-hydroxyvitamin D (p=0.712). - Findings suggest that hypovitaminosis D is associated with increased body sway and an elevated risk for falls and falls-related fractures. Musculoskeletal rehabilitation should include strengthening exercises for the trunk muscles and training of neuromuscular co-ordination and balance.