Introduction and objectives: Myofascial Pain Syndrome (MPS) of the Quadratus Lumborum muscle (QL) is a frequent cause of chronic low back pain. With this study, we aimed to assess the efficacy of ultrasound-guided infiltration with 0.25% levobupivacaine and 40.ßmg triamcinolone for MPS of the QL. Methods: Observational and retrospective study of participants submitted to ultrasound-guided infiltration of the QL muscle from January 1, 2015 to June 31, 2019. Pain intensity was assessed using the five-point pain Numeric Rating Scale (NRS): pre-intervention, at 72.ßhours, 1 month, 3 months and 6 months post-intervention. Additional data collected were demographic characteristics, opioid consumption, and adverse effects. Results: We assessed 90 participants with mean age of 55.2 years. Sixty-eight percent of participants were female. Compared to the pre-intervention assessment, there was an improvement in pain at 72.ßhours (Mean Difference [MD.ß=.ß3.085]; 95% CI: 2.200...3.970, p.ß