Human lumbar spines usually consist of five vertebrae; however, some individuals have vertebral anomalies with four or six lumbar vertebrae because of the lumbarized first sacral vertebra (lumbarization) or the sacralized fifth lumbar vertebra (sacralization), respectively. These vertebral anomalies are called lumbosacral transitional vertebrae (LSTV). This study was performed to determine the prevalence of LSTV and to measure their cross-sectional areas of vertebral canal and dural sac in Koreans using magnetic resonance imaging. We evaluated 9709 patients, comprising of 5570 male and 4139 female subjects. The prevalence of LSTV in our study population was 3.77%, with a higher incidence of lumbarization than sacralization (p<0.05). The prevalence of lumbarization was 2.27%, with a higher rate in men (60.2%), while the incidence of sacralization was 1.50%, with a higher rate in women (62.7%). Differences between the groups were compared separately for the L4~L5 level which represents the level of lumbarization, and the L5~S1 level which represents the level of sacralization. When the vertebral canal cross-sectional areas (VCCSA) were compared between the normal and LSTV groups, the size of the vertebral canal was found to be smaller in the lumbarization group than in the normal group at the L4~L5 level (p<0.001), while there was no significant difference in VCCSA between the normal and LSTV groups at the L5~S1 level. However, when the dural sac cross-sectional areas were compared between the normal and LSTV groups, the size of the dural sac was larger in the LSTV group than in the normal group at the L5~S1 level (p<0.001). These results could be clinically used as one of the parameters for evaluating lumbar vertebral canal stenosis with LSTV in Koreans.