We report a rare case of polymicrogyria diagnosed at 27 weeks' gestation on ultrasound examination and associated with cytomegalovirus (CMV) infection. The ultrasound finding suggesting this diagnosis was the direct visibility of the overfolded cortical ribbon. The cerebral surface was clearly visible because of a markedly enlarged pericerebral space associated with micrencephaly secondary to CMV infection. Bilateral opercular dysplasia was also present. Very few sonographic markers of infectious fetopathy were observed other than periventricular cysts located behind both ventricular horns. Magnetic resonance imaging (MRI) of the fetal brain confirmed the ultrasound findings and also showed the presence of marked micrencephaly, whereas cephalic measurements acquired on ultrasound examination (biparietal diameter and head circumference) were within the normal range. This case emphasizes the complementary roles of sonography and MRI in the prenatal diagnosis of cerebral abnormalities. Moreover, it illustrates the fact that polymicrogyria is easier to diagnose on ultrasound examination during the second trimester, before the development of secondary sulci. Copyright © 2008 ISUOG. Published by John Wiley & Sons, Ltd.