Osteoarthritis (OA) of the knee leads to restrictions of physical activity and ability to perform activities of daily living. Obesity is a risk factor for knee OA and it appears to exacerbate knee pain and disability. The Arthritis, Diet, and Activity Promotion Trial (ADAPT) was developed to test the efficacy of lifestyle behavioral changes on physical function, pain, and disability in obese, sedentary older adults with knee OA. This controlled trial randomized 316 sedentary overweight and obese older adults in a two-by-two factorial design into one of four 18-month duration intervention groups: Healthy Lifestyle Control; Dietary Weight Loss; Structured Exercise; or Combined Exercise and Dietary Weight Loss. The weight-loss goal for the diet groups was a 5% loss at 18 months. The intervention was modeled from principles derived from the group dynamics literature and social cognitive theory. Exercise training consisted of aerobic and strength training for 60 minutes, three times per week in a group and home-based setting. The primary outcome measure was self-report of physical function using the Western Ontario and McMaster University Osteoarthritis Index. Other measurements included timed stair climb, distance walked in 6 minutes, strength, gait, knee pain, health-related quality of life, knee radiographs, body weight, dietary intake, and cost-effectiveness of the interventions. We report baseline data stratified by level of overweight and obesity focusing on self-reported physical function and physical performance tasks. The results from ADAPT will provide approaches clinicians should recommend for behavioral therapies that effectively reduce the incidence of disability associated with knee OA.