BACKGROUND: With accumulating knowledge on osteoarthritis development, the next step is to focus on possibilities for primary prevention. METHODS: In a 2 x 2 factorial design, the effects of a diet-and-exercise program and of oral glucosamine sulfate (double blind and placebo-controlled) on the incidence of knee osteoarthritis were evaluated in a high-risk group of 407 middle-aged women with a body mass index >= 27 kg/m(2) without clinical signs of knee osteoarthritis at baseline (ISRCTN 42823086). Primary outcome was the incidence of knee osteoarthritis, defined as Kellgren & Lawrence grade >= 2, joint space narrowing of >= 1.0 mm, or clinical knee osteoarthritis (clinical and radiographic American College of Rheumatology criteria) after 2.5 years. RESULTS: After 2.5 years, only 10% of all subjects were lost to follow-up, and 17% of all knees showed incident knee osteoarthritis. Accounting for the significant interaction between the interventions, no significant main effect of either intervention was found. Independently, both interventions alone showed indications of reduced knee osteoarthritis incidence (odds ratio [OR] 0.69; 95% CI, 0.39-1.21 for the diet-and-exercise program and OR 0.60; 95% CI, 0.31-1.12 for the glucosamine intervention). These effects were neutralized in subjects receiving both interventions (OR 0.97; 95% CI, 0.55-1.71). CONCLUSIONS: No significant main effects of the diet-and-exercise program and of glucosamine sulfate were found on incident knee osteoarthritis. Nevertheless, this trial provides valuable insights for future trial design for preventive osteoarthritis studies. (C) 2015 Elsevier Inc. All rights reserved.