['Debate surrounds the relative importance of impaired beta-cell secretory function versus insulin resistance in type 2 diabetes. We therefore defined insulin secretion and sensitivity in patients with impaired glucose homoeostasis of varying aetiology and duration.', '126 consecutive patients undergoing an oral glucose tolerance test (OGTT) between 1999 and 2003 were included. Whole-body insulin sensitivity index (ISI) and insulinogenic index derived from the OGTT were determined in 32 healthy controls, 65 type 2 diabetic patients, 15 patients with acromegaly, 10 patients with insulinoma, and 4 patients with HAIRAN syndrome.', 'Median ISI (quartiles Q25-Q75) of healthy controls and of patients with insulinoma were similar (3.5 [2.8-5.6] and 3.2 [1.7-4.2] respectively) but significantly decreased in patients with acromegaly, type 2 diabetes, and HAIRAN syndrome (2.8 [1.8-3.3], 1.9 [1.4-3], and 0.8 [0.6-1.3] respectively). Despite the decrease in ISI, patients with HAIRAN syndrome and acromegaly maintained normal glucose tolerance by adapting insulin secretion as reflected in the insulinogenic index (106.5 [90.4-127.5] and 49 [24.4-89] in HAIRAN and acromegaly respectively, versus 46.9 [27.3-66.7] in controls). In contrast, type 2 diabetic patients failed to adapt and displayed severely hampered insulin secretion (insulinogenic index of 7.6 [3.8-14.7]). Furthermore, the level of the insulinogenic index correlated significantly with duration of diabetes and HbA1c, which was not the case for the ISI. Insulinoma patients had a decreased insulinogenic index (38.7 [32-83.8]), leading to impaired glucose tolerance despite normal ISI.'] Does islets secretory capacity determine glucose homoeostasis in the face of insulin resistance?