The sympathetic nervous system is an important physiological modulator of basal and postprandial energy expenditure. Objective: Our objective was to investigate whether the variability of weight loss attained during hypocaloric dietary intervention is related to individual differences in baseline sympathetic drive and nutritional sympathetic nervous system responsiveness. Participants and Methods: Untreated obese subjects (n = 42; body mass index = 32.1 - 0.5 kg/m2), aged 57 - 1 yr, who fulfilled Adult Treatment Panel III metabolic syndrome criteria participated in a 12-wk weight loss program using a modified Dietary Approaches to Treat Hypertension (DASH) diet. Muscle sympathetic nerve activity (MSNA) was measured by microneurography at rest and in a subset of subjects during a standard 75-g oral glucose tolerance test. Results: Weight loss (6.7 - 0.5 kg) was independently predicted by baseline resting MSNA burst incidence (r = 0.38; P = 0.019), which accounted for 14.3 of the variance after adjustment for age and baseline body weight. Weight loss-resistant subjects in the lower tertile of weight loss (4.4 - 0.3 ) had significantly blunted MSNA responses to oral glucose at baseline compared with successful weight losers (9.6 - 0.8 ). Absolute ? MSNA averaged -7 - 2, -6 - 5, and -3 - 3 bursts per 100 heartbeats at 30, 60, and 90 min after glucose in the weight loss-resistant group. Corresponding values in the successful weight loss group were 9 - 3, 12 - 3, and 15 - 4 bursts per 100 heartbeats (time X group interaction, P = 0.004). Conclusions: These findings indicate that baseline sympathetic drive and nutritional sympathetic responsiveness may be important prognostic biological markers for weight loss outcome.