Background/Aims: This study aims to identify a relationship between body weight (BW), body mass index (BMI) and bone mineral density (BMD) of lumbar spine and femur neck in patients under 50 years old with inactive Crohn’s disease (CD) and ulcerative colitis (UC). Methods: The subjects were CD and UC patients under 50 years old who were regularly followed up in inflammatory bowel disease clinic. The associations between BMD and clinical parameters including BW, height, BMI, disease-onset age, disease-duration, time of disease onset to diagnosis, and biochemical parameters including hemoglobin, c-reactive protein, serum albumin, calcium and phosphorus were evaluated. BMD was measured as Z- score and low BMD was defined less than -1. Results: In enrolled 73 CD (male, 51; 69.9%) and 41 UC (male 21; 51.2%),// 34 (46.6%) and 15 (36.6%) were low BMD, respectively, without difference between two diseases and sex. In CD, mean BW and BMI in low BMD group were lower than those in normal BMD group (59.1 vs. 65.1, p＝0.023 and 20.4 vs. 22.8, p＝0.001). BMI was correlated with BMDs of lumbar spine and of femur neck (0.320, p＝0.006; 0.390, p＝0.001) but BW was correlated only with BMD of femur neck (0.376, p＝0.001). In UC, mean BMI in low BMD group were lower than that in normal BMD group (20.6 vs. 22.1, p＝0.024). Height, BW, and BMI were correlated with BMD of femur neck (0.360, p＝0.021; 0.518, p＝0.001; 0.549, p＜0.001) but not with BMD of lumbar spine. The mean values of other biochemical parameters were not significant different between low and normal BMD in CD and UC. Conclusions: Low BW and BMI could reflect the possibility of low BMD of femur neck in patients under 50 years old with inactive CD and UC.