The Multiple Risk Factor Intervention Trial (MRFIT) revealed an unexpected subgroup finding: an association between diuretic therapy (especially with hydrochlorothiazide) and an increased rate of sudden death in hypertensive men with left ventricular hypertrophy and other ECG abnormalities. The Diuretics, Hypertension, and Arrhythmias Clinical Trial sought to determine whether the finding resulted from random variation or represented a serious toxic response to hydrochlorothiazide.