PURPOSE: Lumbar spinal stenosis (LSS) is a disabling condition associated with narrowing of the spinal canal or vertebral foramina. Paraspinal muscle atrophy and fatty infiltration have been reported in patients with chronic LBP and disc herniation. However, very few imaging studies have examined paraspinal muscle morphology and composition in patients with LSS. The purpose of this study was to investigate the association of paraspinal muscle size, composition and asymmetry with functional status in patients with LSS. METHODS: Thirty-six patients diagnosed with LSS at L4-L5 with neurogenic claudication were included. Paraspinal muscle measurements were obtained from axial T2-weighted MR images, bilaterally, at the level of the superior and inferior vertebral endplates of L5. Muscle measurements of interest included: total cross-sectional area (CSA), functional CSA (FCSA), the ratio of FCSA to CSA (FCSA/CSA) as an indicator of muscle composition, and relative % asymmetry in muscle CSA. The association between muscle parameters and other patient characteristics with function as indicated from Oswestry Disability Index (ODI) scores and pain interference status was investigated. RESULTS: Greater multifidus muscle fatty infiltration (e.g., lower FCSA/CSA) and lower psoas relative CSA were associated with lower function (higher ODI and pain interference scores) in univariable and multivariable analyses. There was no association between the different muscle parameters and stenosis severity or back or leg pain duration or severity. CONCLUSIONS: Our findings suggest an association of multifidus muscle fatty infiltration and psoas muscle size with functional status in patients diagnosed with LSS. Future prospective studies are needed to evaluate whether such muscle parameters are associated with prognosis and functional recovery following surgical treatment.