The target of this work was the determination of a quantifiable measurement of the ventricular dilatation with an index during the ultrasonographic supervision of intraventricular hemorrhages (IVH) in premature infants. An investigation by cerebral ultrasonography through the fontanelles was performed in 63 non hypotrophic prematures. Thirty-one children (mean gestational age 32 +/- 1.8 weeks) had an IVH. All had ventricular dilatation at one time during evolution. The 32 other children (mean gestational age 34 +/- 2 weeks) showed normal ultrasonographic data and a ventricular index between 0.15 and 0.25 (mean 0.23 +/- 0.02). Post-hemorrhagic ventricular dilatation can be classified into 3 stages: slight dilatation: ventricular index between 0.26 and 0.30; moderate dilatation: ventricular index between 0.31 and 0.40; severe dilatation: ventricular index greater than 0.40. The use of a ventricular index allows precise following of the ventricular dilatation during the supervision of intraventricular hemorrhages.