Key words: lumbar stabilization exercise, multifidus muscle size, low back pain. Low back pain is one of the most common conditions in mankind. Therefore, it is needed an exact diagnostic and the most effective problem solving method. There is an evidence of dysfunction in deep lower back and abdominal muscles in low back pain patients. Accordingly, the aim of this study was to define the influence of lumbar stabilization exercise for chronic low back pain, and to examine L4- L5 multifidus and transversus abdominis muscles size. The objectives of our study: to determine and compare normal and chronic low back pain patients ranges of L4- L5 multifidus and transversus abdominis muscles size; to evaluate deep lower back and abdominal muscles function before and after lumbar stabilization exercise; to estimate chronic low back pain changes duaring stabilization exercise. The methods of our research: ultrasound imaging, the instrument of pressure bio- feedback „Stabilizer“, Oswestry Low Back Pain Disability Index Questionnaire, the analysis of mathematical statistics. This work consists of two studies with different participants. 16 healthy individuals and 16 chronic low back pain patients were studied in ultrasound imaging measurement. 12 chronic low back pain patients (4 women and 8 men) were participated in lumbar stabilization exercise program. Results: L4- L5 multifidus muscle size was larger (27 %) in males than in females, transversus abdominis muscle size was larger (17... [to full text]