In mice of normal weight and with diet-induced obesity, a high-fat, low-carbohydrate ketogenic diet (KD) causes weight loss, reduced circulating glucose and lipids, and dramatic changes in hepatic gene expression. Many of the effects of KD are mediated by fibroblast growth factor 21 (FGF21). We tested the effects of KD feeding on ob/ob mice to determine if metabolic effects would occur in obesity secondarily to leptin deficiency. We evaluated the effect of prolonged KD feeding on weight, energy homeostasis, circulating metabolites, glucose homeostasis, and gene expression. Subsequently, we evaluated the effects of leptin and fasting on FGF21 expression in ob/ob mice. KD feeding of ob/ob mice normalized fasting glycemia and substantially reduced insulin and lipid levels in the absence of weight loss. KD feeding was associated with significant increases in lipid oxidative genes and reduced expression of lipid synthetic genes, including stearoyl-coenzyme A desaturase 1, but no change in expression of inflammatory markers. In chow-fed ob/ob mice, FGF21 mRNA was elevated 10-fold compared with wild-type animals, and no increase from this elevated baseline was seen with KD feeding. Administration of leptin to chow-fed ob/ob mice led to a 24-fold induction of FGF21. Fasting also induced hepatic FGF21 in ob/ob mice. Thus, KD feeding improved ob/ob mouse glucose homeostasis without weight loss or altered caloric intake. These data demonstrate that manipulation of dietary macronutrient composition can lead to marked improvements in metabolic profile of leptin-deficient obese mice in the absence of weight loss.