OBJECTIVES: The thermal water of the City of Szigetvár, discovered in 1966, has a temperature of 62 oC when it reaches the surface and contains alkali-bicarbonatec with sodium chloride. The main objective is to explore the potential improvement in the quality of life and the effect of the thermal water as compared to warm tap water (placebo). It is assumed that the water with organic material content has a therapeutical effect. METHODS: Our research focuses on the therapeutic effect of this thermal water on patients suffering from rheumatic diseases, especially cox- and gonarthrosis,  in a double–blind study. Sample size was n=75.  The treatment of 5 days /patient is performed in Szigetvár spa in two randomly selected groups, using two tubs, and lasting 30 minutes each time.  Every patient is given tangentor and bath tub treatment altogether 15 times. The methods used for the collection of data include the VAS, physical examination, document analysis and the Western Ontario and McMaster Universities Arthritis Index (WOMAC).  Data were processed with the Microsoft Office Excel 2003 and Windows SPSS 20.0  softwares, and the results were tested with the Mann-Whitney, the Wilcoxon and the khi-square tests. RESULTS: Our data have shown a significant improvement (p=0.035) in knee extension in the group treated with thermal water in contrast with the group treated with tap-water. The difference in life qualities was also significant measured by a Likert scale (p=0.014). CONCLUSIONS: The thermal water of the City of Szigetvár significantly improved the health status of patients suffering from rheumatic diseases in knee extension.