T article reported a study of the relationship of body-mass index (BMI) to mortality using data from the American Cancer Society’s Cancer Prevention Study II. The magnitude of the cohort is quite significant. This was a prospective study of more than 1 million adults in the United States. It is rare that such a large volume of information is available from any source, public or private. This study tried to avoid the problems where other studies of BMI and mortality have been criticized. These problems include failing to identify smokers and those with a history of disease as possible confounding factors. Failure to exclude or identify these groups have been cited as a potential fatal flaw in the observation in other studies of the so-called ‘‘J-shaped’’ mortality curve, where the most lean subjects have increased mortality. Subjects from the American Cancer Society Cancer Prevention Study II were used for the cohort. About 1.2 million subjects have been enrolled since 1982 and followed through 1996. The average age at enrollment was 57 years, and all enrollees were at least 30 years old. About 1 million subjects qualified for this study.