Low bone mineral density (BMD) is common in patients with Crohn's disease; however, the pathogenesis of bone loss and risk factors for osteoporosis are not established. Our aim was to evaluate the clinical, dietary, and nutritional determinants of BMD in Crohn's disease. A cross-sectional analysis of 117 patients with Crohn's disease was undertaken. All patients underwent a clinical and dietary evaluation including assessment of nutritional state and life-style. BMD was measured at the hip and lumbar spine by dual-energy x-ray absorptiometry; and z scores obtained by comparison with age- and sex-matched normal values for the healthy UK population. Multiple regression analysis was used to assess associations between BMD and potential risk factors, allowing for possible confounding variables. Thirteen (11%) patients had osteoporosis (z score < −2), with osteopenia (z score < −1, > −2) in a further 34 (29%). Patients with jejunal disease had significantly lower BMD at the spine (P = 0.03) and femoral neck (P = 0.02) than those with disease at other sites. Mean BMD was significantly lower at the hip of patients with previous bowel resection (diff in means = 0.53, 95% CI −0.97, −0.08, P = 0.02), but type of surgery was not significant. Active disease, menstrual history, diet, level of physical activity, and smoking were not associated with low bone mass. At the lumbar spine, body weight (P < 0.0001), male sex (P < 0.0001), and current prednisolone use (P < 0.02) were independently predictive of low bone mass. Body weight (P < 0.0001), male sex (P < 0.0001), and cumulative steroid dose (P = 0.02) were predictive at the femoral neck. The major determinants of BMD in Crohn's disease are body weight, current steroid use, and cumulative steroid dose. Men with Crohn's disease are at greatest risk of osteoporosis, with jejunal involvement and previous bowel resection also contributing to the low bone mineral density.