Data from large and small clinical trials reflect major differences in the pathophysiology, treatment, and prognosis of left ventricular (LV) systolic and diastolic dysfunction. These studies also indicate that medical therapy can benefit patients with LV dysfunction regardless of whether or not they are symptomatic. Because the descriptive term congestive heart failure does not provide for these important distinctions, a new classification of LV dysfunction has been developed in which patients with LV dysfunction are categorized on the basis of normal or abnormal systolic function. This classification is based on a simple assessment of LV function, it is applicable to patients without symptoms, and it reflects differences in treatment and prognosis. Those with clinically significant LV systolic dysfunction (ie, an LV ejection fraction <40%) benefit from therapy whether or not they have symptoms of heart failure. Those with LV dysfunction and a normal LV ejection fraction (ie, diastolic dysfunction) also benefit from medical therapy. Annual mortality is higher in those with systolic dysfunction than in those with diastolic dysfunction, but within each of these categories mortality is higher in those with symptoms than in those without. This classification can be useful in the diagnosis and treatment of individual patients as well as in epidemiologic surveys designed to assess medical practice patterns.