Resting energy expenditure (REE) was measured in 112 morbidly obese adults prior to elective gastric bypass surgery. The patients studied ranged from 157 to 327% of ideal body weight. Standard nutritional assessment indices (serum total protein, albumin, total iron binding capacity, hematocrit, and white blood cell count) were within normal limits. REE was estimated by the Harris-Benedict formula using both current weight and ideal weight. Measured REE was significantly less than expected (p less than 0.01) using current weight and significantly greater than expected (p less than 0.01) when ideal weight was used as the standard. Linear regression analysis between standard indices that reflect resting metabolic rate in normal adults and measured REE in study patients did not demonstrate sufficient correlation to be clinically useful in this patient population. Standard surgical therapy may result in highly variable weight loss in this population if the wide range of resting energy expenditure and the consequential variability in individual caloric deficits is not considered. Standard predictors do not identify those patients likely to be unsuccessful with a given weight loss regimen.