Animals that lack the hormone leptin become grossly obese, purportedly for 2 reasons: increased food intake and decreased energy expenditure (thermogenesis). This review examines the experimental evidence for the thermogenesis component. Analysis of the data available led us to conclude that the reports indicating hypometabolism in the leptin-deficient ob/ob mice (as well as in the leptin- receptor-deficient db/db mice and fa/fa rats) derive from a misleading calculation artefact resulting from expression of energy expenditure per gram of body weight and not per intact organism. Correspondingly, the body weight-reducing effects of leptin are not augmented by enhanced thermogenesis. Congruent with this, there is no evidence that the ob/ob mouse demonstrates atrophied brown adipose tissue or diminished levels of total UCP1 mRNA or protein when the ob mutation is studied on the inbred C57BL/6 mouse background, but a reduced sympathetic nerve activity is observed. On the outbred Aston mouse background, brown adipose tissue atrophy is seen, but whether this is of quantitative significance for the development of obesity has not been demonstrated. We conclude that leptin is not a thermogenic hormone. Rather, leptin has effects on body temperature regulation, by opposing torpor bouts and by shifting thermoregulatory thresholds. The central pathways behind these effects are largely unexplored.