Oat is promoted as a low-glycaemic index food. Our aim was to measure the effect of the fat content of oat in glycaemia and insulinaemia and compare it with the effect of wheat. The study design was a double-blind, randomised cross-over study with four treatment segments. Eight healthy males attended four study sessions in which porridge made from four different raw materials was consumed: rolled oat (O), defatted rolled oat (DO), rolled whole wheat (W) or rolled whole wheat with added oat fat (WF). Available carbohydrate content was analysed enzymatically, and was adjusted to 50 g in each test meal. Fat content per meal was either 6·1 g (O, WF) or approximately 1·9 g (W, DO). Venous plasma glucose, insulin and triacylglycerol concentrations were measured at 0, 30, 60, 90, 120 and 180 min. All products caused a rapid increase in glucose with a peak at 30 min, where W and WF were significantly higher than O (0 = 0·006). Also insulin peaked at 30 min (no differences). A 4·2 g difference in fat content between O and DO or W and WF did not result in any significant differences in their glycaemia or insulinaemia. W and O did not differ in their overall glycaemic or insulinaemic responses. The removal of two-thirds of oat fat did not affect the postprandial plasma triacylglycerol concentration. The present study shows that neither the glycaemic nor the insulinaemic response to rolled oat is affected by the fat content, and that rolled wheat differed from rolled oat in terms of peak glucose concentration only.