Objective. To study the possibility and feasibility of using terahertz-modulated infrared radiation in the treatment of patients with gonarthrosis, including those with concomitant chronic lower limb venous insufficiency and to develop a specific treatment procedure. Subjects and methods. A total of 92 patients were examined and treated. The patients matched for age, gender, and main clinical manifestations were divided into a study group and a control one. All the patients received disease-modifying anti-rheumatic drugs, therapeutic exercises, including those in a swimming pool. In addition to the basic treatment, the patients in the study group had terahertz-modulated infrared radiotherapy. Before and after treatment, all the patients underwent a physical examination; their complaints were studied; pain severity was assessed by a visual analog scale (VAS) for pain; lower limb temperature was measured by infrared beam thermography; and microcirculation in the affected knee joint area was analyzed by laser Doppler flowmetry. Results. Analysis of posttreatment changes in clinical manifestations in the study group revealed a significant reversion of clinical manifestations, such as knee joint pain occurring at the end of the day or in the first half of the night (from 42.6 to 15.3%), as well as pain occurring after mechanical load and reducing at rest (from 72.3 to 25.5%). The changes in mean VAS scores in the study group (from 6.1±0.7 to 2.1±0.6) were significantly pronounced (p<0.01) compared with those in the controls (from 6.6±0.7 to 4.3±0.4). Infrared beam thermography established that the study group had a decrease in the thermal asymmetry value between varicose veins and adjacent tissues, indicating a hemodynamic improvement in the lower extremity vessels (by 1.57±0.07 °C in the study group (p<0.05) versus 0.91±0.04 °C in the control group (p<0.05). The ongoing package of rehabilitation measures assisted in reducing the temperature difference between the affected vein area and adjacent tissues. Conclusion. The incorporation of terahertz-modulated infrared radiation into a therapeutic complex was found to have a positive impact on the clinical symptoms of the disease, the state of local hemodynamics and microcirculation, which substantially enhances the efficiency of therapy.