The aim of this study is to assess the prevalence low bone mass among girls with adolescent idiopathic scoliosis (AIS) and their siblings. The subjects of this study were Saudi Arabian girls with AIS. Patients had their weight and height measured to calculate their body mass index (BMI). Clinical examination and investigations were done to rule out any other cause of scoliosis. All had bone mineral density (BMD) measurement of hip area and the spine using DEXA scan, Hologic Inc. Patients with a BMD of < -2.6 was taken as osteoporotic and those between < -1 and -2.5 was taken as osteopenic for analysis. As control subjects, siblings of the patients with normal spine had their BMI calculated and BMD measurement done. We were able to analyze the data of 32 girls with an average age of 18.42 +/- 5.71 (14-26) years with mean BMI of 17.7 +/- 0.69 (16.5-18.5) kg/M2. Analysis of the scans of the hip revealed that 62.5% of the patients were osteoporotic with BMD of 0.837 (0.697-0.936) +/- 0.04, T-score -3.8 +/- 0.56 (-2.6 to -3.9) and Z-score. Nine (28.1%) were osteopenic with BMD of 0.768 +/- 0.15 (0.638-0.878), mean T-score of -1.6 (-1.1 to 2.5) and Z-score -3.5 +/- 0.63 (-2.9 to -3.9). Analysis of BMD of the spine showed similar results. In comparison to the scoliotics, girls with normal spine had higher BMI and BMD which was statistically significant at P < 0.001. T- and Z-score was also lower in scoliotic girls in comparison with girls with normal spine significant at P < 0.001 (CI 95%). Our study indicates that the scoliosis causes osteopenia and osteoporosis among girls while their siblings with normal spine remain with normal bone mass.