Objectives. To explore differences in the composition of the human psoas major muscle in the autopsy cases, with respect to different levels of its origin and degenerative changes of the lumbar spine. Furthermore, the aim was to identify the changes of the psoas major muscle in the levels L3/L4, L4/L5 i L5/S1 of the intervertebral discs, with regard to degenerative changes of the lumbar spine, Lasègue test and physical disability, in patients with low back pain syndrom. Material and methods. Muscle tissue samples for immunohistochemical and Western Blot analysis were collected from male cadavers on four levels (from the first to fourth lumbar vertebra) of the muscle origin. Tissue samples were analysed using monoclonal antibodies against different myosin heavy chain isoforms. Patients with low back pain syndrom were measured using magnetic resonance imaging. Patients were tested using Straight Leg Raising Test and physical disability was assessed with Roland–Morris and Oswestry disability questionnaires. Results. Immunohistochemical analysis showed equal proportion of slow and fast type of the muscle tissue. Autopsy cases with present degenerative changes of the lumbar spine show bigger cross-sectional area of the type I and type IIA muscle fibres when compared to those without degenerative changes. In autopsy cases with degenerative changes in the spine, muscle fibres of all three types have bigger cross-sectional area in the left muscle when compared to the right muscle. In comparison to controls, patients with chronic low back pain syndrome have increased cross-sectional area of the psoas major muscle. In the group of patients with the low back pain, those with present degenerative changes in the lumbar spine have smaller cross-sectional area compared to low back pain patients without degenerative changes. There is negative correlation between physical disability due to pain and size of the cross-sectional area of the psoas major muscle. Conclusion. The psoas major muscle shows hypertrophic changes in patients with symptoms of low back pain syndrome, as well as in the autopsy cases with the degenerative changes of the lumbar spine. With increased pain in the lumbar region the psoas muscle shows signs of decreased activity, but its postural and dynamic roles remain unchanged regardless to degenerative changes of the lumbar spine