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70 fetuses (10 to 37 weeks) and 30 full term infants were examined by ultrasound (U.S.). Anatomic correlations were made with frontal, axial and sagittal sections of 43 fetal and 3 neonatal brains. At 12 weeks gestational age (w.g.a.) only the inter hemispheric fissure is seen by U.S. The sylvian fissure (21 w.g.a.), the callosal sulcus (21 w.g.a.), the parieto-occipital sulcus (25 w.g.a.), the calcarine fissure (25 w.g.a.), the cingulate sulcus (26 w.g.a.) and the collateral sulcus (25-27 w.g.a.) are visualized rather late with in-utero U.S. The other sulci are more difficult to see. The peripheral location of these sulci contributes to the difficulty encountered in their visualization. The morphology of the sylvian fissure is quite characteristic on U.S. imaging and can be used to estimate the gestational age of the fetus. Compared to the embryological development there is a 2-4 weeks delay between the first infolding of the brain and the visualization of a sulcus by U.S. Abnormal sulcal patterns can be recognized based on the normal appearance for each gestational age. Sulcal anomalies are quite specific in holoprosencephaly, lissencephaly, micropolygyry, schizencephaly, agenesis of the corpus callosum. Silhouetting of the sulci may occur if the parenchymal echogenicity is sufficiently increased that the sulci no longer stand out (ischemia, tumors, encephalitis). Thickening of the sulci occurs in subdural hematomas, external hydrocephalus, meningitis and toxoplasmosis.