Following onset of acute cardiogenic pulmonary edema in 21 patients, increases in hematocrit, plasma protein concentration, and colloid osmotic pressure were associated with decreases in plasma volume. Accordingly, there was a loss of hypo-oncotic fluid into the extravascular spaces. Following treatment with oxygen, furosemide, and morphine sulfate and reversal of clinical and radiographic signs of pulmonary edema, declines in hematocrit, plasma protein concentration, and colloid osmotic pressure were associated with increases in plasma volume. Hypo-oncotic edema fluid was therefore reabsorbed into the vascular compartment. The concept that acute heeart failure with pulmonary edema is associated with an increase in intravascular volume is therefore not supported. To the contrary, there is a reduction of blood volume during acute pulmonary edema. During reversal of acute pulmonary edema with diuresis, there was re-expansion rather than contraction of blood volume.