Context: Leptin affects neurogenesis, neuronal growth, and viability. We previously reported that leptin supplementation increased gray matter (GM) concentration in the anterior cingulate gyrus (ACG), cerebellum, and inferior parietal lobule, areas that are also involved in food intake. Objective: The aim of this study was to report the changes in brain structure at different states of leptin supplementation. Design: We conducted a nonrandomized trial. Setting and Patients: We studied three adults with congenital leptin deficiency due to a mutation in the leptin gene. Intervention: Patients received treatment with recombinant methionyl human leptin, with annual 11- to 36-d periods of treatment withholding followed by treatment restoration over 3 yr. Main Outcome Measures: GM concentration (by voxel-based morphometry analysis of magnetic resonance scans) was correlated with body mass index (BMI) and leptin supplementation. Results: Annually withholding leptin supplementation for several weeks increased BMI and reversed the original effects of leptin in the cerebellum and ACG. The changes in the ACG were consistent with an indirect effect of leptin mediated through increased BMI. In the cerebellum, where leptin receptors are most dense,GMchanges appeared to be direct effects of leptin. Leptin restoration did not lead to recovery of GM in the short term but did lead to an unexpected GM increase in the posterior half of the left thalamus, particularly the pulvinar nucleus. Conclusion: These findings provide the first in vivo evidence of remarkably plastic, reversible, and regionally specific effects of leptin onhumanbrain morphology. They suggest that leptinmayhave therapeutic value in modulating plasticity-dependent brain functions.