Spondylolisthesis is the slippage of one vertebral body on an adjacent level, and occurs commonly at the lumbosacral junction in children. Many radiographic measurement instruments have been described to predict progression and need for intervention. We evaluated the reliability of eight common grading instruments. Four raters reviewed 30 lateral radiographs of the lumbar spine in patients with spondylolisthesis. Each rater measured each film twice, and had mean individual correlation coefficients of at least 0.76 (range, 0.76-0.91). Only three measurements had interobserver correlations greater than 0.75 (slip percentage, Meyerding's grade, and sacral inclination), which corresponded to excellent reliability. For intraobserver reliability, six measurements had correlations greater than 0.75 (all except kyphosis angle and lumbar index), indicating excellent agreement. Slip percent, Meyerding's grade, and sacral inclination had excellent interobserver agreement and intraobserver agreement.