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The objective was to determine the effect of a single dose of espresso caffeinated coffee, decaffeinated coffee, or water on glucose tolerance and insulin sensitivity in people with type 2 diabetes mellitus. Eighteen participants who were habitual coffee drinkers, were studied using a random-order cross-over design. After a fasting blood sample participants consumed either a double-shot black espresso coffee, decaffeinated coffee, or hot water. The main outcomes were area under the curve (AUC) glucose and insulin, and insulin sensitivity (Matsuda index) during a 75 g oral glucose tolerance test (OGTT) performed one hour later. Other outcomes were change in glucose and insulin and also the insulinogenic index (IGI) and disposition index (DI). AUC glucose was marginally different between beverages ( P =.06) being greater following caffeinated coffee than water, mean difference 104 mmol/L/180 min (95% CI 0.1 to 198.1, P =.031), or decaffeinated coffee, mean difference 92.1 mmol/L/180 min (95% CI −1.9 to 186.1, P =.055). There was no difference in AUC insulin ( P =.87) or insulin sensitivity ( P =.47), nor in change in glucose or insulin over the hour following beverage consumption. There was a marginal difference in IGI between beverages ( P =.097) with coffee having a lower incremental increase in insulin/glucose than water ( P =.037) though no difference between coffee and decaffeinated coffee ( P =.54) and no difference in DI ( P =.23). Black espresso coffee in people with type 2 diabetes mellitus results in a marginally greater excursion of glucose during a following OGTT compared with water or decaffeinated coffee. This effect does not appear to be mediated by changes in insulin sensitivity.