The National Cholesterol Education Program Diet vs. a diet lower in carbohydrates and higher in protein and monounsaturated fat
In Fall 2004, Dr. Agatston and his colleagues published a paper in the Archives of Internal Medicine (Aude et al. 2004) showing that subjects on The South Beach Diet™ lost twice as much weight as subjects following the National Cholesterol Education Program low-fat diet.
See the full study here.

A randomized, controlled clinical trial to evaluate the efficacy of a modified carbohydrate diet for reducing body weight and fat in overweight and obese men and women
In Spring 2005, a second study was presented at the Experimental Biology 2005 meeting of the Federation of American Societies for Experimental Biology. It showed that participants who followed the principles of The South Beach Diet™ lost twice as much weight as those following a portion-controlled, low-fat plan, and also showed greater losses in body fat. In addition, they experienced significant reductions in triglycerides and their ratio of total/HDL cholesterol.
Read a summary of the study, or see the abstract for more information.