BACKGROUND:Because of the high density of fat, high-fat diets are perceived as likely to lead to increased bodyweight, hence health-care providers are reluctant to recommend them to overweight or obese individuals. We assessed the long-term effects of ad libitum, high-fat, high-vegetable-fat Mediterranean diets on bodyweight and waist circumference in older people at risk of cardiovascular disease, most of whom were overweight or obese. METHODS:PREDIMED was a 5 year parallel-group, multicentre, randomised, controlled clinical trial done in primary care centres affiliated to 11 hospitals in Spain. 7447 asymptomatic men (aged 55-80 years) and women (aged 60-80 years) who had type 2 diabetes or three or more cardiovascular risk factors were randomly assigned (1:1:1) with a computer-generated number sequence to one of three interventions: Mediterranean diet supplemented with extra-virgin olive oil (n=2543); Mediterranean diet supplemented with nuts (n=2454); or a control diet (advice to reduce dietary fat; n=2450). Energy restriction was not advised, nor was physical activity promoted. In this analysis of the trial, we measured bodyweight and waist circumference at baseline and yearly for 5 years in the intention-to-treat population. The PREDIMED trial is registered with ISRCTN.com, number ISRCTN35739639. FINDINGS:After a median 4·8 years (IQR 2·8-5·8) of follow-up, participants in all three groups had marginally reduced bodyweight and increased waist circumference. The adjusted difference in 5 year changes in bodyweight in the Mediterranean diet with olive oil group was -0·43 kg (95% CI -0·86 to -0·01; p=0·044) and in the nut group was -0·08 kg (-0·50 to 0·35; p=0·730), compared with the control group. The adjusted difference in 5 year changes in waist circumference was -0·55 cm (-1·16 to -0·06; p=0·048) in the Mediterranean diet with olive oil group and -0·94 cm (-1·60 to -0·27; p=0·006) in the nut group, compared with the control group. INTERPRETATION:A long-term intervention with an unrestricted-calorie, high-vegetable-fat Mediterranean diet was associated with decreases in bodyweight and less gain in central adiposity compared with a control diet. These results lend support to advice not restricting intake of healthy fats for bodyweight maintenance. FUNDING:Spanish Government, CIBERobn, Instituto de Salud Carlos III, Hojiblanca, Patrimonio Comunal Olivarero, California Walnut Commission, Borges SA, and Morella Nuts.