Background context: The lumbar multifidus muscle provides an important contribution to lumbar spine stability, and the restoration of lumbar multifidus function is a frequent goal of rehabilitation. Currently, there are no reliable and valid physical examination procedures available to assess lumbar multifidus function among patients with low back pain. Purpose: To examine the inter-rater reliability and concurrent validity of the multifidus lift test (MLT) to identify lumbar multifidus dysfunction among patients with low back pain. Study design/setting: A cross-sectional analysis of reliability and concurrent validity performed in a university outpatient research facility. Patient sample: Thirty-two persons aged 18 to 60 years with current low back pain and a minimum modified Oswestry disability score of 20%. Study participants were excluded if they reported a history of lumbar spine surgery, lumbar radiculopathy, medical red flags, osteoporosis, or had recently been treated with spinal manipulation or trunk stabilization exercises. Outcome measures: Concurrent measures of lumbar multifidus muscle function at the L4-L5 and L5-S1 levels were obtained with the MLT (index test) and real-time ultrasound imaging (reference standard). Methods: The inter-rater reliability of the MLT was examined by measuring the level of agreement between two blinded examiners. Concurrent validity of the MLT was investigated by comparing clinicians' judgments with real-time ultrasound imaging measures of lumbar multifidus function. Results: Inter-rater reliability of the MLT was substantial to excellent (κ=0.75 to 0.81, p≤.01) and free from errors of bias and prevalence. When performed at L4-L5 or L5-S1, the MLT demonstrated evidence of concurrent validity through its relationship with the reference standard results at L4-L5 (rbis=0.59-0.73, p≤.01). The MLT generally failed to demonstrate a relationship with the reference standard results from the L5-S1 level. Conclusions: Our results provide preliminary evidence supporting the reliability and validity of the MLT to assess lumbar multifidus function at the L4-L5 spinal level. Additional research examining the measurement properties and utility of this test should be undertaken before confident implementation with patients.