Patients with chronic heart failure (CHF) have an increased risk for sudden death. This increased risk has been associated with increased QT dispersion (QTd), a reflection of the heterogeneity in ventricular repolarization. Angiotensin-converting enzyme (ACE) inhibitors have been reported to decrease heart size as well as decreasing morbidity and mortality in moderate-to-severe CHF. The aim of this study was to determine if ACE therapy is associated with a decrease in QTd, a marker for increased electrical instability. Ninety-seven patients were evaluated. The normalized QTd after 2 months of ACE therapy decreased from 16 ± 4 to 12 ± 3, a 25% reduction in dispersions. QTd also decreased from 61 ± 14 to 47 ± 12 (P< .001) and QTc dispersions decreased from 71 ± 18 to 52 ± 14 (P< .001). After 2 months of ACE inhibitor therapy, heart rate slowed significantly (RR intervals 765 ± 198 before and 838 ± 186 after ACE). There was a negative correlation between ejection fraction and QTd (r= −0.8;P< .001). The study also found no correlation between ACE level, percent converting enzyme inhibition, and QTd. The effects of ACE therapy appear early on in terms of repolarization changes. The reduction in QTd may explain the reduced sudden death mortality in patients with heart failure who are treated with ACE inhibitor therapy.