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Introduction This study aimed to investigate factors related to fat infiltration in patients with lumbar degenerative diseases (lumbar disc herniation and/or spinal stenosis), examining a wide range of potential risk variables. Materials and methods We studied consecutive adult patients who underwent posterior lumbar spinal surgery for degenerative diseases at our hospital between July 2013 and June 2017. Preoperative magnetic resonance imaging was used to evaluate the presence or absence of fat infiltration at the level of the L4-5 lumbar paraspinal muscles using Kjaer’s evaluation method. Patients without fat infiltration (0%–10%) were rated as grade 0, grade 1 for moderate fat infiltration (10%–50%), and grade 2 for severe fat infiltration (>50%). Patients were then divided into two groups: Group A (without fat infiltration, grade 0) and Group B (with fat infiltration, grade 1 or 2). Detailed patient clinical data were collected and analyzed. Results A total of 205 consecutive patients were enrolled; 54 (26.3%) patients were assigned to Group A and 151 (73.7%) to Group B. Logistic regression analysis revealed two independent risk factors for fat infiltration of the lumbar paraspinal muscles: female sex and older age (P < .001). Discussion Fat infiltration of the lumbar paraspinal muscles is reported to be associated with the development of pain and dysfunction of the lumbar region and postoperative complications of spinal instrumented fusion surgery. To the best of our knowledge, no previous studies have identified female sex and older age as independent risk factors for fat infiltration in the lumbar paraspinal muscles using multivariate analysis. Conclusions Female sex and older age were independent risk factors for fat infiltration in the lumbar paraspinal muscles. The results of the current study may provide useful information for the study of preventive measures for fat infiltration.