Background Magnetic resonance imaging (MRI) is the diagnostic modality of choice in defining soft tissue compromise of the spinal canal. Purpose To evaluate the reliability of postoperative MRI in the determination of level and side of lumbar spinal decompression surgery, investigated by two reviewers, in different levels of training and specialization. Material and Methods Postoperative MR images of 86 patients who underwent spinal decompression (single level, n = 70; multilevel, n = 16; revision decompression, n = 9) were reviewed independently by an experienced musculoskeletal radiologist and a fourth-year orthopedic surgery resident. The level (single or multiple) and side of previous surgical decompression were determined and compared to the surgical notes. We examined factors that may have influenced the reliability, including demographics, type of surgical decompression, use of a drain, and time interval from surgery to MRI. Results Significantly fewer levels were correctly determined by the resident (77/86 cases, 89.5%) compared with the radiologist (84/86 cases, 97.7%) ( P = 0.014). The resident interpreted significantly more MR images incorrectly in cases where a drain was used (n = 8; P < 0.001). Re-decompression cases were interpreted incorrectly significantly more often by both the radiologist (n = 2, P = 0.032) and the resident (n = 4, P = 0.014). Conclusion Determination of the level and side operated on in previous lumbar spinal decompression surgery on MRI has a high reliability, especially when performed by a musculoskeletal radiologist. However, this reliability is decreased in cases involving surgical drainage and same-level revision surgery.