Background: QT dispersion (QTd) is the difference between the longest and the shortest QT interval in the standard 12-lead ECG. QTd may be a measure of spatial dispersion of repolarization in the myocardium, and may be influenced by antihypertensive treatment. Objective: To study the effects of losartan in comparison to amlodipine on QTd in mildly to moderately hypertensive patients, and to assess correlations of reduced QTd with echocardiographic left ventricular mass (LVM). Methods: Twenty-eight adult patients with mild to moderate hypertension were randomly assigned to a double-blind treatment either with losartan 50 mg titrated to losartan 50 mg-hydrochlorothiazide 12.5 mg or amlodipine 5 mg titrated to amlodipine 10 mg. Blood pressure and echocardiographic LVM were measured before and after 16 weeks of treatment. QTd was measured by two independent examiners blinded to patient data, study phase and treatment. Results: Systolic and diastolic blood pressure were significantly reduced in the losartan and amlodipine groups. Treatment with losartan significantly reduced LVM (from 311 ± 31 to 252 ± 16 g, mean ± SE; p < 0.01), cardiac mass index and QTd (from 42.3 ± 5.4 to 27.8 ± 2.2 ms, mean ± SE; p < 0.05). Conclusion: Losartan significantly reduced QTd after 16 weeks of treatment in patients with mild to moderate hypertension. This reduction was not significantly correlated with the reduction in LVM.