Congestive heart failure (CHF) is characterized by frequent exacerbation of symptoms, often leading to hospitalization. Knowledge of these factors could help prevent clinical deterioration. This investigation was a prospective evaluation of the acute precipitants of CHF conducted as a substudy of the Randomized Evaluation of Strategies for Left Ventricular Dysfunction Pilot Study, a randomized trial of candesartan alone, enalapril alone, candesartan plus enalapril, and metoprolol or placebo in 768 patients over 43 weeks. Investigators systematically reported all episodes (n = 323) of worsening CHF symptoms. The factors associated with acute worsening of CHF symptoms were excessive sodium intake, noncardiac causes (notably pulmonary infections), arrhythmias, and the use of study medications, antiarrhythmic agents, and calcium channel blockers. The authors conclude that a wide variety of factors, many of which are avoidable, are associated with exacerbation of CHF.