Ten patients with gestational proteinuric hypertension were studied with a Swan-Ganz thermodilution haemodynamic catheter before, during and after plasma volume expansion. Five patients were treated with dihydralazine before volume expansion and five after volume expansion. Before treatment all patients had a low pulmonary capillary wedge pressure (PCWP), low cardiac index (CI) and high systemic vascular resistance (SVR). Following volume expansion the PCWP and CI increased, the SVR decreased but the blood pressure (BP) was unchanged. Administration of dihydralazine following volume expansion led to a decrease in PCWP, an increase in CI and a decrease in SVR and BP. Dihydralazine alone caused an increase in heart rate, PCWP, and CI, and a decrease in SVR and BP. Volume expansion, by increasing CI and decreasing SVR, may be of therapeutic benefit in the severely hypertensive pregnant patient with a low cardiac index.