In a controlled prospective randomized study we evaluated the effectiveness of intraarticular injected hyaluronic acid regarding the changes in the parameters dynamometry, synovial oxygen partial pressure, intraarticular temperature and the knee joint function (Lequesne score). We examined 18 patients with osteoarthritis in both knee joints (Kellgren classification II or III according to x-ray) in right/left comparison. The evaluated parameters were measured before and after a 5-week treatment phase with 5 intraarticular injections (one injection per week). With regard to the efficacy results changes in dynamometric measurement for flexion and extension (for all angular velocities) and Lequesne-score indicate a marked improvement after treatment with hyaluronic acid. The differences between treated and untreated knee were significant. In extension the p-value was 0.0008 and in flexion p = 0.0015. The p-value of the Lequesne-index between the verum and control group was p = 0.0002. After treatment values for oxygen pressure were higher than at baseline and for the intraarticular temperature a small decrease at the end of the treatment phase was documented. The intraarticular injection of hyaluronic acid is effective and safe in the treatment of patients with gonarthrosis. The treatment resulted in a functional improvement of the knee joint and had a good pain-lowering effect in patients suffering from gonarthrosis.