OBJECTIVE: The objective of this study is to compare serum levels of FKN and SFRP-4 in patients with normal glucose tolerance (NGT), impaired glucose tolerance (IGT), and type 2 diabetes mellitus (T2DM). METHODS: A total of 152 patients presented to the endocrinology outpatient clinic of our hospital were included in the study. Eighty-two patients with a history of T2DM were assigned to the T2DM group. IGT (n = 34) and NGT (n = 36) groups included the patients who received oral glucose tolerance test outcomes. RESULTS: Serum FKN levels were significantly higher in the IGT and T2DM groups compared to the NGT group (p < 0.001 and p < 0.001, respectively). Serum SFRP-4 levels were significantly higher in the T2DM group compared to the IGT and NGT groups (p = 0.001 and p = 0.004, respectively). A significant correlation was observed between FKN and fasting glucose levels. SFRP-4 was significantly correlated with fasting glucose, HbA1c, and triglyceride levels. CONCLUSION: To our knowledge, increased FKN levels in patients with IGT were demonstrated for the first time in this study. The results of our study support the opinion that FKN and SFRP-4 may contribute to the pathogenesis of T2DM (Tab. 1, Fig. 3, Ref. 23).