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Background: Some patients with inflammatory bowel disease have reduced bone mineral density, but the risk factors for osteoporosis in these patients are unclear. Methods: To evaluate the effect of smoking and other lifestyle factors on bone mineral density in patients with inflammatory bowel disease, we studied 67 patients with ulcerative colitis, 78 with Crohn's disease, 7 with indeterminate colitis, and 73 healthy control subjects. Bone mineral density of the lumbar spine and the proximal femur was measured, using dual-energy X-ray absorptiometry. Measures of smoking and other lifestyle factors were assessed in an interview. Results: The female ex-or current smokers with inflammatory bowel disease (n = 38) had lower age-and sex-adjusted Z-scores of bone mineral density than the female patients who had never smoked (n = 34) (Z-scores in the lumbar spine, -0.277 (1.283) (mean (standard deviation)) and 0.487 (1.056), respectively; p = 0.008; and in the femoral neck, -0.626 (1.055) and -0.013 (1.019); p = 0.015). These differences were not explained by the type or treatment of the disease, the menstrual history, or the use of estrogen preparations. In male patients no differences in bone mineral density were found between ex-or current smokers and non-smokers. Coffee drinking and alcohol consumption were not associated with bone mineral density in these patients. Conclusions: Smoking is associated with low bone mineral density in women with inflammatory bowel disease. This association is not related to the body mass index, the medical treatment, or the type of disease.