Background. The aim of our study was to compare the clinical effectiveness of conventional and water-filtered infrared irradiation treatment in osteoarthritis of the knee. Material and Method. The tests involved 30 people with osteoarthritis of the knee. 10 of them were treated with a conventional infrared irradiation lamp, 20 with an infrared-A irradiation lamp (Hydrosun). Before and after treatment each of the patients was tested for joint mobility, tensometric strength of flexion and extension of knee joints, and subjective pain. Results. After 10 days, both groups showed improvement in the assessed parameters, and the decrease in the subjective pain level was statistically significant. In the group treated with the Hydrosun lamp, the improvement in the mobility of the knee joints was also statistically significant. Conclusion. The results obtained confirm that the best results were achieved in treatment of the osteoarthritis in the knee with the Hydrosun water-filtered infrared lamp.