Patients with acute coronary syndromes (ACS) show prolongation of QT interval duration and its dispersion (QTd). Prolongation of QTd has been associated with inhomogeneity of ventricular recovery times and high arrhythmyogenic potential. Previous studies have shown that preservation of the ischaemic preconditioning is associated with shorter QTd. Sulfonylureas may inhibit cardiac ischaemic preconditioning. The effect, however, of the previous treatment with sulfonylureas on QTd in patients with ACS has not been studied so far. This cross-sectional study examined the effect of the previous antidiabetic treatment on QTd in patients with type 2 diabetes during ACS. A total of 150 patients with ACS (myocardial infarction: n = 120; unstable angina: n = 30) admitted to the coronary care unit of our hospital were studied. Three groups of patients were examined: patients without diabetes (n = 60); patients with type 2 diabetes treated with sulfonylureas alone or in combination with metformin (n = 50); and patients treated with insulin alone or in combination with metformin (n = 40). Standard 12-lead ECG recordings at admission to the coronary care unit were obtained. QT interval duration and QTd were measured using ECG analysis software. At admission, QTd was not different between diabetic and non-diabetic patients (72.1 +/- 21.7 vs. 78.4 +/- 21.3 msec, p = 0.13, respectively). Similarly, the values of the above interval were also not different between patients with type 2 diabetes treated with sulfonylureas and insulin (73.8 +/- 23.9 vs. 70.1 +/- 18.5 msec, p = 0.55, respectively). It is concluded that the previous treatment with either sulfonylureas or insulin does not affect QTd in patients with type 2 diabetes and ACS.