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Sequential study of magnetic resonance images of the lumbar spine.To determine the variation in position of the conus medullaris in a living adult population without spinal deformity.Cadaveric studies have shown that the position of the conus medullaris in the adult most commonly is located at L1-L2. There have been several studies documenting the changing level of the conus throughout infancy and childhood, but there is no detailed study that documents the range of conus positions in a living adult population without spinal deformity.The T1-weighted, midline, sagittal, spin-echo magnetic resonance imaging studies of 504 patients were assessed to identify the tip of the conus medullaris. This location was recorded in relation to the upper, middle, or lower third of the adjacent vertebral body or the adjacent intervertebral disc.The study group consisted of 231 men and 273 women with a mean age 46 years (range, 16-85 years). Most patients were being examined for low back pain. Patients with spinal deformity were excluded. The mean conus position was the lower third of L1 (range, middle third of T12 to upper third of L3). The variation in conus positions followed a normal distribution. No significant difference in conus position was seen between male and female patients or with increasing age.The distribution of conus location in a large adult population was shown to range from the middle third of T12 to the upper third of L3.