FGF21 is a critical metabolic regulator, pivotal for fasting adaptation and directly regulated by PPARalpha in rodents. However, the physiological role of FGF21 in man is not yet defined and was investigated in our study. Serum FGF21 varied 250-fold among 76 healthy individuals and did not relate to age, gender, body mass index (BMI), serum lipids, or plasma glucose. FGF21 levels had no diurnal variation and were unrelated to bile acid or cholesterol synthesis. Ketosis induced by a 2 day fast or feeding a ketogenic diet (KD) did not influence FGF21 levels, whereas a 74% increase occurred after 7 days of fasting. Hypertriglyceridemic nondiabetic patients had 2-fold elevated FGF21 levels, which were further increased by 28% during fenofibrate treatment. FGF21 circulates in human plasma and increases by extreme fasting and PPARalpha activation. The wide interindividual variation and the induction of ketogenesis independent of FGF21 levels indicate that the physiological role of FGF21 in humans may differ from that in mice.