OBJECTIVES: This study sought to determine whether circadian patterns in ventricular arrhythmias (VAs) occur in a current primary prevention defibrillator (implantable cardioverter-defibrillator [ICD]) population. BACKGROUND: Cardiovascular events, including VAs, demonstrate biorhythmic periodicity. METHODS: We tested for deviation from the previously described occurrences of a morning peak, early morning nadir, and peak on Mondays in ICD therapies using generalized estimating equations and Student t tests. All hypothesis tests were performed in the entire cohort of patients with VAs as well as pre-specified subgroups. RESULTS: Of 811 patients with an ICD, 186 subjects experienced 714 ICD therapy episodes for life-threatening VA. There was no morning (6 am to 12 pm) peak in therapies for the entire cohort or any subgroups. The overall cohort and several subgroups had a typical early morning (12 am to 6 am) nadir in therapies, with significantly less than 25% of therapies occurring during this 6-h block (all p < 0.05). A significant peak in therapies on Mondays occurred only in patients not on beta-blocker therapy (22% of events for the week, p = 0.029). CONCLUSIONS: In the SCD-HeFT (Sudden Cardiac Death in Heart Failure Trial) population, the distribution of life-threatening VA failed to show a typical early morning peak or increased VA events on Mondays. A typical early morning nadir was seen in the entire cohort. An increased rate of events on Mondays was found in the subgroup of subjects not on beta-blocker therapy. These findings may indicate suppression of the neurohormonal triggers for VA by current heart failure therapy, particularly the use of beta-blockers in heart failure.