Objective.—To assess a typical commercial trekking cohort for cognitive impairment after gradual ascent to 5100 m.

Methods.—We performed a prospective, controlled, repeated-measures study within a trekking expedition to Nepal. A sample of expedition participants was studied; 36 were enrolled and 26 completed all testing. Additional normative data were sourced from sea level studies. Participants underwent cognitive assessment before travel with written, verbal, and computerized tests, then within 24 hours of arrival at 5100 m after an 18-day ascent from 400 m. Changes in performance in 6 written and 7 computerized tests were analyzed at an individual and group level using paired t tests. Effect size analysis was performed for individual performance.

Results.—No individual demonstrated significant cognitive impairment at 5100 m. The subject group performed significantly better than the normative population in 3 of the 6 written tests. The group performed worse at 5100 m in 1 written test (digit span forwards, P < .01) and better in 2 written (digit-symbol substitution, P < .01; trail-making test, part B, P < .05) and 1 computerized test (monitoring test reaction time, P < .01). Performance was more variable in the written than the computerized tests.

Conclusions.—Gradual ascent to high altitude causes no significant cognitive impairment in the majority of individuals. Computerized testing produced less variable results than written testing, but logistical difficulties are likely to preclude widespread use of such technology in the field.