The aim of this study was to explore, in healthy children, the arterial blood pressure response to a 3-week stay at high altitude (4200 m). An auscultatory automatic ambulatory pressuremeter was used to avoid undue environmental influence on the measurement. The blood pressure was monitored three times in a group of ten boys, aged 10.5 (CI 0.9 years): at sea level (control values), at an altitude of 2100 m after at least 24 h of acclimatization and after at least 24 h at 4200 m altitude. Each period of monitoring extended over 24 h with 10-min intervals between successive measurements. Arterial blood pressure was evaluated separately for the night and day periods. Nocturnal recordings revealed an increase with altitude in systolic as well as in the diastolic blood pressure. Because of the technique used to gather data, this is thought to have represented an independent effect of altitude without interference from the medical environment or diurnal activity.