Rapid ascent to altitude risks the development of acute mountain sickness. This study demonstrates changes in peripheral capillary filtration coefficient and renal protein loss in subjects suffering from various degrees of mountain sickness after passive ascent to 4559 m. Capillary filtration coefficient of the calf capillary bed, measured by computer-based multistep strain gauge plethysmography, increased significantly after 23.5 h at altitude when symptoms were most severe: 4.45 (2.76-6.03) to 6.31 (3.86-11.07) ml min(-1) per 100 g of tissue mmHg(-1), median (range) (P < 0.02). Urinary albumin excretion was increased after one night at altitude from 1.1 (0.6-1.5) to 2.45 (1.0-6-8) mg of albumin per mmol of creatinine (P < 0.05). These results demonstrate simultaneous leakage of a peripheral capillary bed to fluid measured by strain gauge plethysmography, and renal albumin leak, and suggest a systemic process of increased capillary leakage for different-sized molecules caused by rapid exposure to hypobaric hypoxia.