Association of pneumothorax and hypotension with intraventricular haemorrhage. To test the hypothesis that acute hypotension resulting from pneumothorax would be associated with severe brain injury (grade 3 or 4 intraventricular haemorrhage), 67 very low birthweight (VLBW) infants of 32 weeks' gestation or less with respiratory distress syndrome and pneumothorax were studied. Thirty six had pneumothorax associated with systemic hypotension and 31 had pneumothorax with normal blood pressure. The groups were similar in gestational age and severity of their respiratory distress syndrome. Thirty two of 36 of infants with pneumothorax associated with hypotension (89%) had grade 3 or 4 intraventricular haemorrhage. This percentage was significantly greater than the percentage for infants with pneumothorax and normal blood pressure (three of 31, 10%). The risk ratio for grade 3 or 4 intraventricular haemorrhage for infants with pneumothorax associated with hypotension was 9.8 compared with neonates with pneumothorax and normal blood pressure. These observations are consistent with the hypothesis that systemic hypotension and cerebral hypoperfusion are important factors leading to intraventricular haemorrhage in VLBW infants. 