Background/Aims: We investigated the associations among body mass index (BMI), insulin resistance, and β-cell function in Korean patients newly presenting with type 2 diabetes. Methods: In total, 132 patients with new-onset type 2 diabetes mellitus were investigated. A standard 75-g oral glucose tolerance test was performed, and the indices of insulin secretion and insulin resistance were calculated. Results: A higher BMI was associated with higher homeostasis model assessment values for insulin resistance (HOMAIR), homeostasis model assessment of β-cell function (HOMA-β), and insulinogenic index as well as lower levels of insulin sensitivity index composite (ISIcomp) and disposition index (DI). In multiple regression models, BMI had independent positive associations with HOMA-IR, ISIcomp, and HOMA-β and inverse associations with the DI. Conclusions: Our results showed that BMI had independent positive associations with indices of insulin resistance and an inverse association with β-cell function adjusted for insulin resistance in Korean patients newly presenting with type 2 diabetes.