This report describes the occurrence of rapid progression of hydrocephalus after discharge from the nursery in four of 48 infants who had had previous arrest of progression of post-hemorrhagic hydrocephalus, and at least partial resolution of ventriculomegaly. This later-onset hydrocephalus occurred at a mean age of seven months; the most consistent presenting clinical feature was rapid head growth. Three of the four infants required a ventriculo-peritoneal shunt and the fourth was treated with acetazolamide, with apparent resolution of the hydrocephalus. Newborn infants with post-hemorrhagic hydrocephalus should be followed carefully throughout the first year for prompt detection of later hydrocephalus.