Surgery in the lithotomy position can provoke ischaemic lesions in the lower leg. We assessed lower leg oxygen saturation using near-infrared spectroscopy (NIRS) in 42 patients undergoing urinary tract surgery. Lower leg perfusion pressure was calculated as the difference of mean arterial pressure to pressure in an air bag supporting the lower leg and the hydrostatic pressure difference from the level of the lower leg to the heart. During elevation of the lower leg for 25 (3-65) min (median and range), mean arterial pressure decreased from 100 (73-125) to 77 (53-112) mmHg and the lower leg perfusion pressure dropped from 103 (80-122) to 21 (-6-65) mmHg, corresponding to a reduction in oxygen saturation of the medial gastrocnemius muscle from 68% (40-100%) to 58% (20-96%) (P < 0.01). The results demonstrate significant desaturation of the calf muscles during surgery in the lithomy position.