Osteoarthritis (OA) is one of the most common form of chronic diseases in the adult population, and approximately 800 000 people in Sweden (population of 9 million people) suffer from this disease, which causes joint pain, stiffness, swelling, loss of function and sick leave. There is increasing evidence that exercise therapy is effective for patients with knee OA. However, there is little knowledge on long-term effects and what type of exercises dosage is most effective. In this present clinical trial, which is a Nordic collaborative project with researchers from different research groups in Sweden and Norway, we intend to investigate short and long-term effects comparing high dosage exercise therapy (each treatment lasting 80 to 90 min) vs. a relatively low dosage exercise program (25 to 30 min) in patients diagnosed with knee OA. Patients in both intervention groups receive three treatments a week for 12 weeks making a total of 36 treatments. Primary outcome is pain-ratings and function (The Knee Injury and Osteoarthritis Outcome Score (KOOS)). Secondary outcome concerns various health-ratings and objective functional tests. Regarding predictors for outcome, we plan also to investigate different psychosocial variables as well as patient's beliefs regarding exercise.