Background: In Denmark, thousands of infants and children require general anaesthesia annually. Hypotension during general anaesthesia might reduce cerebral blood flow and oxygen delivery to the brain. Safe lower limits of blood pressure are ill defined. The Hypotension in Paediatric Populations Observational study objective was to assess blood pressure in Danish children during general anaesthesia. Methods: This study is a prospective observational multicentre study. Primary outcomes were mean arterial blood pressures in children aged 0-12 years. Lowest mean arterial blood pressure, intervention thresholds to increase blood pressure and type of intervention were secondary outcomes. Premature infants and children scheduled for cardio-thoracic surgery were excluded. Blood pressures were measured by oscillometry or invasively. Results: In total, 726 patients were included. In children < 1 year, median arterial pressure was 51 mm Hg, (interquartile range; 43-58) and increased to 58 mm Hg (interquartile range; 52-65) for 12-year-old children. In 32 patients, 49 actions were taken to modulate blood pressure. Pre-induction blood pressures were recorded for 29%. Conclusion: This study presents pragmatic, multicentre, prospectively collected observations of blood pressure in children undergoing general anaesthesia in usual practice. In the youngest infants, variability in blood pressure appears to be large. Measurement of blood pressure is recommended during every general anaesthesia and in children of all ages. Safe ranges of blood pressure remain to be defined.