Germinal matrix hemorrhage leading to intraventricular hemorrhage results from an excessive pressure gradient between the blood-vessel lumen and the surrounding brain tissue. This gradient may be caused by increased intravascular pressure or--as this paper hypothesises--by decreased tissue pressure. Brain-tissue pressure depends on water content, and it decreases when water moves out of the brain, as occurs in conditions associated with dehydration, plasma hyperosmolality or hypotension. Such conditions occur frequently in premature infants, which may explain why intraventricular hemorrhage develops with a delay after birth. Its coincidence in time with postnatal head-shrinkage and its association with plasma hyperosmolality, hyperosmolar alkali therapy and hypotension strengthen the hypothesis of a cause and effect relationship between reduced cerebral tissue pressure and intraventricular hemorrhage.