STUDY DESIGN: This is a retrospective study. PURPOSE: To compare the morphometric parameters of the psoas major and lumbar multifidus muscles in lumbar spinal stenosis (LSS) patients exhibiting different functional performance. LSS refers to symptoms related to size reduction of the lumbar spinal canal; however, the degree of stenosis is poorly related to symptom severity and functional impairments. Morphometric changes in the paraspinal muscles have been correlated with chronic lower back pain in previous studies. However, correlations between the functional performance of LSS patients and the morphometric changes in paraspinal muscles have seldom been reported. METHODS: Sixty-six LSS patients without mechanical back pain or segmental instability were enrolled in the study. A review of their medical records and MRI images was performed. Morphometric parameters, including the fatty infiltration (FI) of the lumbar multifidus muscle as well as the relative cross-sectional area (RCSA) of the psoas major and lumbar multifidus muscles, were measured. Subjects were divided into high and low functional performance groups according to their Japanese Orthopedic Association (JOA) scores. RESULTS: The male LSS patients exhibited a larger psoas RCSA than the female patients, whereas the older patients exhibited a smaller psoas RCSA and higher multifidus FI than the younger patients. LSS patients in the high functional performance group exhibited a significantly larger psoas RCSA and lower multifidus FI. CONCLUSION: The psoas RCSA and multifidus FI can be used as predictive factors for functional performance in LSS patients.