Although acute hypertensive pulmonary edema is sometimes regarded as the most severe form of heart failure, at the peak of symptoms, hearts perform well above resting levels and cannot be said to be failing. Another characteristic of the condition, the rapidity of its onset and reversal when properly treated, suggests positive feedback as a causal mechanism. It is proposed that the syndrome results from a feedback loop with increased sympathetic tone as the efferent output, increased pulmonary vascular pressure as the stimulus to increased sympathetic tone, and positive feedback occurring because elevated sympathetic tone constricts systemic veins, thereby transferring blood from peripheral veins to the pulmonary vasculature. Evidence for the proposed mechanism derives from all the empirical treatments that have evolved. All remove blood from the pulmonary circuit, and all but the oldest, bloodletting, do so by transferring blood from the pulmonary circuit to the peripheral veins.