Objective: Serum concentrations of the hepatokine fibroblast growth factor (FGF) 21 are elevated in obesity, type-2 diabetes, and the metabolic syndrome. We asked whether FGF21 levels differ between subjects with metabolically healthy vs. unhealthy obesity (MHO vs. MUHO), opening the possibility that FGF21 is a cross-talker between liver and adipose tissue in MUHO. Furthermore, we studied the effects of chronic FGF21 treatment on adipocyte differentiation, lipid storage, and adipokine secretion. Methods: In 20 morbidly obese donors of abdominal subcutaneous fat biopsies discordant for their whole-body insulin sensitivity (hereby classified as MHO or MUHO subjects), serum FGF21 was quantified. The impact of chronic FGF21 treatment on differentiation, lipid accumulation, and adipokine release was assessed in isolated preadipocytes differentiated invitro. Results: Serum FGF21 concentrations were more than two-fold higher in MUHO as compared to MHO subjects (457±378 vs. 211±123pg/mL; p<0.05). FGF21 treatment of human preadipocytes for the entire differentiation period was modestly lipogenic (+15%; p<0.05), reduced the expression of key adipogenic transcription factors (PPARG and CEBPA,-15% and-40%, respectively; p<0.01 both), reduced adiponectin expression (-20%; p<0.05), markedly reduced adiponectin release (-60%; p<0.01), and substantially increased leptin (+60%; p<0.01) and interleukin-6 (+50%; p<0.001) release. Conclusions: The hepatokine FGF21 exerts weak lipogenic and anti-adipogenic actions and marked adiponectin-suppressive and leptin and interleukin-6 release-promoting effects in human differentiating preadipocytes. Together with the higher serum concentrations in MUHO subjects, our findings reveal FGF21 as a circulating factor promoting the development of metabolically unhealthy adipocytes.