The syndrome of plethora without splenomegaly, leukocytosis, or thrombocytosis was described by Gaisböck in 1905. It was reevaluated in 25 patients through the study of numerous hematologic and biochemical parameters. Statistically significant findings included mild obesity; elevation of blood pressure (especially diastolic); decrease in plasma volume with relative increase in red cell count, hemoglobin, hematocrit, viscosity of blood, elevation of plasma proteins, serum cholesterol, triglycerides, uric acid, and plasma renin; and increased excretion of urinary sodium. The reduction in plasma volume seemed related to the elevation of the diastolic blood pressure, and favorable through temporary therapeutic results were produced by some antihypertensive agents. The elevation of serum cholesterol, triglycerides, and uric acid, the increased blood viscosity, and the elevated plasma renin were reminescent of the biochemical changes observed in some instances of hypertension and atherosclerotic disease. They may explain why patients with the Gaisböck's syndrome belong to the high risk group of hypertensive individuals who often develop cardiovascular complications. While its long-term effects remain to be evaluated, treatment with nondiuretic antihypertensive drugs may help reduce the incidence of these complications.