The purpose of the present study is to detect by means of MRI any structural changes in the brain and their correlation with the clinical history of climbers who have ascended to extremely high altitudes without supplementary oxygen. Clinical history, neurological examinations and brain MRI were obtained from a group of 26 climbers who ascended to over 7000 m without supplementary oxygen, and the results were compared with a control group (n = 21) of healthy subjects. All the MRI studies were carried out between 26 days and 36 months after return to sea level. Significant neuropsychological disorders were experienced by all climbers during the ascent with residual neurobehavioural impairment after returning to sea level in 58% of them. The neurological examination was normal in all subjects. Almost half of the climbers showed MRI abnormalities (46%). Characteristic signal patterns of cortical atrophy were detected in five subjects. Periventricular hyperintensity lesions in the T2-weighted images were observed in other five climbers. Both types of lesions were found in two subjects. These pathological findings did not correlate with age, sex, clinical symptoms, maximal altitude reached, or length of exposure to extreme altitude. The exact long-term pathological significance of these MRI abnormalities is as yet unknown.