The purpose was to determine the efficacy of therapeutic ultrasound in patients with chronic low back pain. Thirty-one patients, age 38-77, with low back pain lasting more than three months and the intensity of pain on visual analogue scale at least 50 mm, are randomly divided in two groups. Ultrasound is applied on the lumbar paravertebral muscle in 16 patients and in 15 patients the machine was not switched on. All patients also underwent kinesitherapy. Pharmacological treatment was not changed during the research (except the possibility of using paracetamol as the "rescue drug"). Following parameters were measured at the beginning and at the end of the research: pain intensity on the visual analogue scale/mm, modified Schober measure/cm, patient's and physician's global assessment of treatment efficacy (1-5 scale). The intensity of pain in the ultrasound group before the treatment was 82,7+/-14,0 and after the treatment 79,8+/-12,2 (p<0,05). The intensity of pain in the placebo group before the treatment was 81,7+/-12,1 and at the end of the treatment 78,9+/-12,1 (p>0,05). The value of the modified Schober measure for the ultrasound group were 5,7+0,8 cm vs. 5,8+/-0,9 cm (p>0,05) and in the placebo group were 5,4+/-0,9 cm vs. 5,6+/-1,0 cm (p>0,05). There was no significant statistical difference between ultrasound and placebo group regarding the efficacy of the treatment (patients p>0,05, physicians p>0,05). Therapeutic ultrasound was effective in decreasing the pain intensity in this research, but showed no improvement regarding the functional ability of the lumbar spine in patients with chronic low back pain.