Monosomy 1p36 is an increasingly recognized chromosomal anomaly. We describe two patients with monosomy 1p36 who had brain abnormalities detected on prenatal ultrasound. The first patient was ascertained prenatally with ultrasound abnormalities, including ventriculomegaly, a single umbilical artery, a unilateral club foot, a ventricular septal defect, and intra-uterine growth retardation. Amniocentesis showed a normal karyotype. A postnatal MRI showed moderate to severe non-obstructive hydrocephalus, bilateral colpocephaly, and abnormal myelination of the anterior limb of the internal capsule. A postnatal karyotype demonstrated a deletion of 1p36.3 that was not detected prenatally due to low resolution. Molecular studies by array comparative genome hybridization (CGH) identified a terminal deletion of ∼10 Mb. Our second patient was a fetus who had brain abnormalities suggestive of holoprosencephaly identified on prenatal ultrasound. Amniocentesis showed 46,XX,der(1)t(1;20)(p36.1;p12.2), that was found to be maternally inherited. Fetal autopsy demonstrated hydrocephalus, focal polymicrogyria, and cerebellar hypoplasia. However, holoprosencephaly was not confirmed. In addition to describing two patients with monosomy 1p36 who had abnormal brain anatomy on prenatal ultrasounds, we review the literature of other prenatally detected patients with monosomy 1p36 and review brain abnormalities seen both prenatally and postnatally. © 2008 Wiley-Liss, Inc.