Fibroblast growth factor 21 (FGF 21) has been suggested as a predictor for the development of type 2 diabetes mellitus (T2DM).We aimed to determine FGF 21 levels in normoglycaemic (Group 1) and prediabetic (Group 2) first-degree relatives (FDR) of patients with T2DM in comparison with normoglycaemic subjects without a history of T2DM in their FDR (Group 3).There was a significant difference between Group 1, 2, and 3 with respect to plasma FGF 21 concentrations (143.3 ± 93.8, 221.9 ± ± 171.7 and 121.2 ± 119.8 pg/mL, respectively, p = 0.01). FGF 21 levels were significantly increased in prediabetic FDR of patients with T2DM compared to normoglycaemic subjects without a history of T2DM in their FDR (p = 0.02). FGF 21 levels did not differ between normoglycaemic FDR of patients with T2DM and normoglycaemic subjects without a history of T2DM in their FDR (p > 0.05). In the whole group, FGF 21 correlated positively with age (r = 0.31, p = 0.003), BMI (r = 0.38, p < 0.001), systolic blood pressure (r = 0.38, p = 0.001), diastolic blood pressure (r = 0.26, p = 0.02), fasting blood glucose (r = 0.24, p = 0.02), HOMA-IR (r = 0.23, p = 0.03), AUC glucose (r = 0.35, p = 0.001), and AUC insulin (r = 0.32, p = 0.003) and negatively with HDL cholesterol (r = -0.24, p = 0.02) and Matsuda ISI (r = -0.33, p = 0.002). In the regression analysis, BMI was the most predictive factor for FGF 21 levels (beta = 0.41, r2 = 0.17, p < 0.001).We showed that FGF 21 concentrations are increased in prediabetic FDR of patients with T2DM and that there is a significant association between FGF 21 and obesity and insulin sensitivity. (Endokrynol Pol 2016; 67 (3): 260-264).