There is a wide range of disorders in which plasma volume may be contracted. This may either be due to a primary reduction in plasma volume, or secondary to a reduction in the volume of the vascular compartment. In the former there may be a salt and water deficit as the cause or a loss of plasma volume due to a capillary leak. Reduction of the vascular compartment is usually due to increased adrenergic activity reducing the circulatory capacitance volume. Plasma volume is reduced to compensate for the overfilling of the central circulation. The causes and clinical significance of plasma volume contraction are discussed, with particular emphasis on the pathophysiology and implications for therapy. The controversial subject of stress polycythaemia is addressed and a theoretical analysis of its pathophysiology presented.