CONTEXT: Five-year survival rates for early stage colorectal, breast, and prostate cancer currently exceed 90% and are increasing. Cancer survivors are at greater risk for second malignancies, other comorbidities, and accelerated functional decline. Lifestyle interventions may provide benefit, but it is unknown whether long-term cancer survivors can modify their lifestyle behaviors sufficiently to improve functional status. OBJECTIVE: To determine whether a telephone counseling and mailed print material-based diet and exercise intervention is effective in reorienting functional decline in older, overweight cancer survivors. DESIGN, SETTING, AND PARTICIPANTS: Randomized controlled trial of 641 overweight (body mass index > or = 25 and < 40), long-term (> or = 5 years) survivors (aged 65-91 years) of colorectal, breast, and prostate cancer, who were randomly assigned to an intervention group (n = 319) or delayed intervention (control) group (n = 322) in Canada, the United Kingdom, and 21 US states. Individuals were recruited for the Reach out to Enhance Wellness (RENEW) trial from July 1, 2005, through May 17, 2007. INTERVENTION: A 12-month, home-based tailored program of telephone counseling and mailed materials promoting exercise, improved diet quality, and modest weight loss. The control group was wait-listed for 12 months. MAIN OUTCOME MEASURES: Change in self-reported physical function on the Short-Form 36 physical function subscale (score range, 0-100; a high score indicates better functioning) from baseline to 12 months was the primary end point. Secondary outcomes included changes in function on the basic and advanced lower extremity function subscales of the Late Life Function and Disability Index (score range, 0-100), physical activity, body mass index, and overall health-related quality of life. RESULTS: The mean baseline Short-Form 36 physical function score was 75.7. At the 12-month follow-up, the mean function scores declined less rapidly in the intervention group (-2.15; 95% confidence interval [CI], -0.36 to -3.93) compared with the control group (-4.84; 95% CI, -3.04 to -6.63) (P = .03). The mean baseline basic lower extremity function score was 78.2. The mean changes in basic lower extremity function were 0.34 (95% CI, -0.84 to 1.52) in the intervention group compared with -1.89 (95% CI, -0.70 to -3.09) in the control group (P = .005). Physical activity, dietary behaviors, and overall quality of life increased significantly in the intervention group compared with the control group, and weight loss also was greater (2.06 kg [95% CI, 1.69 to 2.43 kg] vs 0.92 kg [95% CI, 0.51 to 1.33 kg], respectively; P < .001). CONCLUSION: Among older, long-term survivors of colorectal, breast, and prostate cancer, a diet and exercise intervention reduced the rate of self-reported functional decline compared with no intervention. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT00303875.