To study the effect of thermal mineral water of Nagybaracska (Hungary) on patients with primary knee osteoarthritis in a randomized, double-blind clinical trial, 64 patients with nonsurgical knee joint osteoarthritis were randomly selected either into the thermal mineral water or into the tap water group in a non-spa resort village. The patients of both groups received 30-min sessions of bathing, 5 days a week for four consecutive weeks. The patients were evaluated by a blind observer immediately before and at the end of the trial using Western Ontario and McMaster Osteoarthritis (WOMAC) indices and follow-up assessment 3 months later. Twenty-seven patients of the 32 patients who received thermal mineral water and 25 of the 32 of those treated with tap water completed the trial. The WOMAC activity, pain, and total scores improved significantly in the thermal mineral-water-treated group. The improvement remained also at the end of the 3-month follow-up. The WOMAC activity, pain, and total scores improved significantly also in the tap water group at the end of the treatment course, but no improvement was detected at the end of the 3-month follow-up period. The treatment with the thermal mineral water of Nagybaracska significantly improved activity, pain, and total WOMAC scores of patients with nonsurgical OA of the knee. Even after 3 months, significant improvement was observed compared to the scores before the treatment or to tap water treatment.