Introduction: There is a wealth of evidence from a number of sources that body weight and the proportion of energy from dietary fat are directly related. However, some randomized controlled trials (RCTs) of low-fat diets versus other types of low-energy diets have found no difference in weight loss between treatment groups. Thus, the overall aim of this systematic review and meta-analysis is to assess the effectiveness of low-fat diets as a means of achieving sustained weight loss, using all available randomized clinical trials. Methods: The Cochrane Library, MEDLINE, EMBASE, BIOSIS and bibliographies will be systematically searched for relevant studies. Key journals will be hand searched and experts approached for additional studies. Interim results: The interim meta-analyses suggest that, in overweight and/or obese clients, ad libitum low-fat diets are not more efficacious than other types of low-energy diets in terms of weight loss, and also that, in the same population, low-energy low-fat diets are not more efficacious than low-energy diets which are not low in fat, in terms of weight loss. However, the studies included in the two interim meta-analyses revealed significant heterogeneity which will be examined in greater detail in the final meta-analysis. Conclusion: From our interim analyses of the data, we conclude that low-fat diets are as efficacious as other types of low-energy diets in promoting weight loss in the overweight and obese. Updates and the final report of this systematic review will be published in the Cochrane Library.