STUDY DESIGN: Sequential study of magnetic resonance imaging (MRI) of the spine for assessing the level of termination of the conus medullaris (CMT) and thecal sac (TST). OBJECTIVES: To determine the variation in CMT and the TST as a function of age and gender in an adult population without spinal deformity and to investigate the relationship between CMT and TST. SUMMARY OF BACKGROUND DATA: It is generally accepted that the conus medullaris terminates in the lower third of L1; however, a wide range of values has been reported in cadaver studies as well as in MRI studies during life. There have been only a few reports on the influence of gender on the CMT, and the influence of age has not been studied at all. There have been few studies on the range of termination of the thecal sac, and the influence of age and gender remain unexplored. METHODS: We reviewed the 3-mm sagittal MR images of 635 patients, including 297 women and 338 men, with a mean age of 49.43 years. The CMT and TST were located in relation to spinal levels and assigned numerical values ranging from 0 to 13. Parametric and nonparametric statistical tests were used for the data analysis. RESULTS: Mean CMT was at the level of the middle third of L1. The range span extended from the lower third of Th11 to the upper third of L3. The CMT displayed a small but significant positive correlation with age and with gender. Mean TST was at the level of the upper third of S2. The range extended from the lower third of L3 to the upper third of S5. The TST was not affected by gender, but age had an effect on TST. CONCLUSIONS: The CMT and TST displayed a wide range of values in our study. We detected small but systematic influences of gender and age on CMT and of age on TST, as well as a positive correlation between CMT and TST. These effects are small in amplitude, but they met all the criteria for statistical significance and have practical value for clinicians, as well as theoretical value for the medical and biologic community.