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Objective: to elucidate the frequency and pathogenesis of myofascial pain syndrome (MFPS) in chronic nonspecific lower back pain (CNLBP) and to optimize the diagnosis and treatment of MFPS in CNLBP. Patients and methods. The investigation covered 121 patients with CNLBP. The patients' mean age was 42.1±10.5 years; the pain duration was 7.9±4.3 months. The possible causes of CNLBP were determined: these were facet joints (FJs); sacroiliac joints (SIJs); skeletal muscles with the development of MFPS; MFPS concurrent with FJs; MFPS concurrent with SIJs. Twenty patients had MFPS only (its mean duration was 5.3±2.2 months; the mean pain intensity scores were 6.5±1.1 on a numerical rating scale). Six patients underwent examinations of open biopsy specimens of the muscle straightening the spinal column; a comparison group consisted of 3 healthy women matched for age and gender. The patients were prescribed therapy with aceclofenac 200 mg/day in combination with tolperisone 450 mg/day and nondrug therapy (cognitive behavioral therapy and kinesio- and ergotherapy). When the treatment was insufficiently effective, ultrasonography of the muscle straightening the spinal column was additionally performed; a local anesthetic was injected into myofascial trigger points (MTPs). Results and discussion. MFPS was a cause of pain syndrome in 63 (52%) patients, while MFPS this was an isolated cause of pain in 20 (16.5%) cases and was concurrent with FJ osteoarthritis in 23 (19%), and with SIJ dysfunction in 20 (16.5%). Muscle ultrasonography in patients with MFPS revealed MTPs, whereas examinations of biopsy specimens of the muscle straightening the spinal column showed no evidence of necrosis, fibrosis, or inflammatory infiltration in the presence of transformation of the myosin phenotype, by increasing the proportion of rapidly fatigued type II muscle fibers. The results of sodium dodecyl sulfate (SDS) gel electrophoresis indicated a decrease in the content of titin and nebulin, the sarcomeric cytoskeletal proteins involved in maintaining muscle contractility. A two-week cycle of therapy with aceclofenac and tolperisone reversed pain syndrome in 5 (25%) of the 20 patients and reduced the intensity of back pain in 15 (75%), but the pain increased during physical exercise and impeded active rehabilitation. The additional administration of anesthetics into MTPs and the continuous intake of aceclofenac and tolperisone in combination with kinesiotherapy could relieve pain syndrome and enhance motor activity. Conclusion. More than half of the patients with CNLBP had MFPS only or concurrent with joint pathology (FS and FJs). The changes found in the back muscle biopsy specimens of patients with CNLBP are potentially reversible and can be reversed during kinesiotherapy.