BACKGROUND/OBJECTIVE: Diet books dominate the New York Times Advice Best Seller list and consumers cite such books as an important source of nutrition information. However, the scientific support for nutrition claims presented as fact (nutrition facts) in diet books is not known. DESIGN/MEASUREMENTS: We assessed the quality of nutrition facts in the best-selling South Beach Diet using support in peer-reviewed literature as a measure of quality. We performed structured literature searches on nutrition facts located in the books' text, and then assigned each fact to 1 of 4 categories (1) fact supported, (2) fact not supported, (3) fact both supported and not supported, and (4) no related papers. A panel of expert reviewers adjudicated the findings. RESULTS: Forty-two nutrition facts were included. Fourteen (33%) facts were supported, 7 (17%) were not supported, 18 (43%) were both supported and not supported, and 3 (7%) had no related papers, including the fact that the diet had been "scientifically studied and proven effective." CONCLUSIONS: Consumers obtain nutrition information from diet books. We found that over 67% of nutrition facts in a best-seller diet book may not be supported in the peer-reviewed literature. These findings have important implications for educating consumers about nutrition information sources.