Seven climbers spent 6 weeks in the East African mountains, at heights of up to 5890 m. The mean protein urine concentration in morning specimens was over 100 mg/100 ml after climbs during the first 12 days but fell to 15 mg/100 ml during subsequent climbs. The highest concentrations (100-300 mg/100 ml) were in 5 climbers with peripheral oedema, 3 with obvious high-altitude illness. The findings implicate the kidney in high altitude acclimatization and illness.