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Assessment of the reliability of standardized magnetic resonance imaging (MRI) interpretations and measurements.To determine the intra- and inter-reader reliability of MRI parameters relevant to patients with intervertebral disc herniation (IDH), including disc morphology classification, degree of thecal sac compromise, grading of nerve root impingement, and measurements of cross-sectional area of the spinal canal, thecal sac, and disc fragment.MRI is increasingly used to assess patients with sciatica and IDH, but the relationship between specific imaging characteristics and patient outcomes remains uncertain. Although other studies have evaluated the reliability of certain MRI characteristics, comprehensive evaluation of the reliability of readings of herniated disc features on MRI is lacking.Sixty randomly selected MR images from patients with IDH enrolled in the Spine Patient Outcomes Research Trial were each rated according to defined criteria by 4 independent readers (3 radiologists and 1 orthopedic surgeon). Quantitative measurements were performed separately by 2 other radiologists. A sample of 20 MRIs was re-evaluated by each reader at least 1 month later. Agreement for rating data were assessed with kappa statistics using linear weights. Reliability of the quantitative measurements was assessed using intraclass correlation coefficients (ICCs) and summaries of measurement error.Inter-reader reliability was substantial for disc morphology [overall kappa 0.81 (95% confidence interval (CI): 0.78, 0.85)], moderate for thecal sac compression [overall kappa 0.54 (95% CI: 0.37, 0.68)], and moderate for grading nerve root impingement [overall kappa 0.47 (95% CI: 0.36, 0.56)]. Quantitative measures showed high ICCs of 0.87 to 0.96 for spinal canal and thecal sac cross-sectional areas. Measures of disc fragment area had moderate ICCs of 0.65 to 0.83. Mean absolute differences between measurements ranged from approximately 15% to 20%.Classification of disc morphology showed substantial intra- and inter-reader agreement, whereas thecal sac and nerve root compression showed more moderate reader reliability. Quantitative measures of canal and thecal sac area showed good reliability, whereas measurement of disc fragment area showed more modest reliability.