The inadequacies of existing methods for expressing the results of weight reduction programs have been discussed and a new reduction index has been derived. The index requires that a target weight be established for each patient at the beginning of every dietary regimen. The target weight may be chosen from standard tables, from “ideal weight” tables, or arbitrarily on the basis of the physician's judgment. The new index is obtained by multiplying the per cent of excess weight loss by the relative initial obesity. Its component terms are a product of two factors: (1) a reduction coefficient, which can be computed at the beginning of the dieting, and (2) the amount of weight lost during the active dieting. The value of the reduction coefficient will characterize the patient's initial obese status. The reduction index will characterize the accomplishment in weight reduction. Individual “success” is considered to have been achieved by patients who obtain a reduction index of 60 or higher. The total results of a dietary regimen may then be expressed by stating the percentage of the patients who obtained reduction indices of 60 or higher. The data obtained in a group of 106 dieting patients have been expressed in terms of existing methods of documenting weight loss, and the use of the new reduction index in this group has been demonstrated. The results of a series of different dietary programs have been compared using the reduction index and other parameters of measurement. It is suggested that the reduction index be adopted as a standard for expressing the data of weight reduction regimens in order that valid comparison can be given to the results of different dietary programs.