We measured the haemodynamic effects of changing from the supine position to the lateral decubitus (lateral) position, and then to the kidney rest lateral decubitus (kidney) position in 12 patients undergoing nephrectomy under isoflurane anaesthesia. Eight control patients undergoing pulmonary surgery remained in the lateral position. The lateral position produced no significant changes. In the kidney position, however, significant reductions occurred in the mean arterial (P < 0.01), right atrial (P < 0.05) and pulmonary artery wedge pressures (P < 0.01). There were also significant reductions in cardiac index (from 3.04 (SD 0.21) to 2.44 (0.26) litre min-1 m-2, P < 0.01) and stroke volume index (from 40 (5) to 31 (5) ml beat-1 m-2, P < 0.01). The systemic vascular resistance index increased significantly (P < 0.05). Cardiac output was probably reduced by a decrease in venous return and an increase in systemic vascular resistance.