Albumin was measured by dipstick tests and immunologically in 24-h and early morning urine (EMU) samples collected from 20 subjects during a high-altitude trek. Each was given acetazolamide (Diamox sustets) or placebo as part of a double-blind trial on the prophylactic use of acetazolamide in acute mountain sickness (AMS). At the highest altitudes, albuminuria was six times greater in those on placebo (p less than 0.001) and was related to the clinical features of AMS (p less than 0.01) and arterial oxygen tension (p less than 0.001). Urine dipsticks tests for proteinuria were also an index of AMS, but were inaccurate. The proteinuria is probably due to renal hypoxia, which causes increased glomerular permeability, reduced tubular readsorption, or both. The reduction in the clinical features of AMS achieved with acetazolamide therapy is also associate with improved renal function.