During the 1981-90 decade, 31 unilateral grade III intraventricular hemorrhages (III IVH), with controlateral grade I or grade II, and 65 bilateral grade III IVH were detected by brain ultrasound studies in the neonatal intensive care unit of Port-Royal. Varied hemorrhages were present on day 1 in 66% of unilateral III IVH and 72% of bilateral III IVH, and in all cases by day 4. Bilateral grade III was reached only between days 4 and 8 in 32% of cases. Death rate was 35% in unilateral III IVH and 66% in bilateral III IVH. Median dates of observation were: days 2-3, clots in the third ventricle and cisterna magna; days 29-35, disappearance of clots in lateral ventricles; days 2-4, onset of ventricular dilatation; days 15-20, maximal dilatation; days 22-27, onset of regressive dilatation; day 54-3 months, maximal regression of dilatation. The total number of true hydrocephalus was 3/31 in unilateral III IVH and 12/65 in bilateral III IVH. The 20 survivors after unilateral III IVH had 17 regressive dilatations (10 spontaneous and 7 with acetazolamide), 1 hydrocephalus treated by ventriculo-peritoneal shunt (VPS), 2 regressions with late hydrocephalus (2 VPS). The 21 survivors after bilateral III IVH had 19 regressive dilatations (6 spontaneous and 13 with acetazolamide), and 2 hydrocephalus (2 VPS). A late limited dilatation reappeared between 3 and 9 months of age in infants with neonatal periventricular leukomalacias.