Context: The role of adipose triglyceride lipase (ATGL) in intermediate substrates metabolism has not been fully elucidated in humans. Objective: Our objective was to evaluate the consequences of ATGL deficiency on body fat distribution, insulin sensitivity, fatty acids metabolism, and energy substrate utilization. Design and Setting: Body composition and organ fat content were measured by bioimpedance and H-1 nuclear magnetic resonance spectroscopy; heart glucose metabolism by [F-18]deoxyglucose positron emission tomography and insulin sensitivity and beta-cell function by oral glucose tolerance and 2-step euglycemic-hyperinsulinemic clamp. Lipolysis ([H-2(5)] glycerol turnover) and indirect calorimetry were evaluated at fasting, after oral glucose load, during the clamp, and also during an iv epinephrine infusion. These metabolic investigations were carried out during hospitalization. Patients: Three patients affected by neutral lipid storage disease with myopathy (NLSDM) due to homozygosity for loss-of-function mutations in the ATGL gene and 6 sex-, age-, and body mass index-matched controls were studied. Results: As expected, NLSDM patients showed diffuse, although heterogeneous, fat infiltration in skeletal muscles associated with increased visceral fat. Although heart and liver were variably affected, fat content in the pancreas was increased in all patients. Compared with healthy controls, NLSDM patients showed impaired insulin response to glucose possibly related to the severe pancreatic steatosis, preserved whole-body insulin sensitivity, and a shift toward glucose metabolism in the heart. Fasting nonesterified fatty acid concentrations as well as basal lipolytic rates and the antilipolytic effect of insulin were normal in NLSDM patients, whereas the lipolytic effect of norepinephrine was impaired. Finally, no significant abnormality in the respiratory quotient was noted in NLSDM patients. Conclusions: In humans, ATGL has a remarkable effect on cellular lipid droplet handling, and its lack causes both perivisceral, skeletal muscle, and pancreas fat accumulation; in contrast, the impact on whole-body insulin sensitivity and fatty acid metabolism is minor.