Abstract: Objective: This study investigates the feasibility and diagnostic value of L1 vertebral computed tomography (CT) attenuation for the opportunistic screening of osteoporosis via routine abdominal CT scans. Methods: Data from 1117 patients who underwent quantitative computed tomography (QCT) and abdominal CT were analyzed retrospectively. The bone mineral density (BMD) and CT attenuation values of the L1 and L2 vertebrae were measured, and patients were divided into three groups according to the BMD measured using QCT: osteoporosis, osteopenia, or normal BMD. The correlation between the two measurements was analyzed. The L1 attenuation CT value was measured by two observers at two time points separated by at least two weeks. Bland–Altman plots were used to assess the agreement between the predicted and measured QCT values. Receiver operating characteristic (ROC) curves were generated to determine the optimal diagnostic thresholds and the area under the curve (AUC). Results: The L1 and L2 vertebral CT attenuation values correlated well with the QCT BMD values (correlation coefficients of 0.956 and 0.902; all P < 0.001). There was excellent intra- and interobserver agreement in the L1 vertebral attenuation CT value measurements (ICC = 0.995 and 0.985) for the two observers. An analysis of the Bland–Altman plot showed that there was good agreement between the predicted and measured QCT BMD values. The CT and mean CT attenuation values from the L1 and L2 vertebrae predicted osteoporosis and normal BMD with high accuracy. The CT attenuation values of the L1 and L2 vertebrae predicted osteoporosis and normal BMD with high accuracy but no significant differences. The AUC of the L1 and L2 vertebrae CT attenuation values and combined mean CT attenuation values of L1 and L2 for predicting osteoporosis were 0.982, 0.977, and 0.984, respectively, and those for predicting normal BMD were 0.969, 0.964, and 0.970, respectively. The optimal thresholds of the L1 vertebrae for predicting osteoporosis and normal BMD were 106.33 HU and 149.33 HU, respectively. Conclusion: L1 vertebral CT attenuation values on abdominal CT scans are highly reliable and valuable for opportunistic osteoporosis screening. Thus, the L1 vertebral attenuation CT value can be used for opportunistic osteoporosis screening.