Objective To describe prenatal magnetic resonance imaging (MRI) findings of dural sinus malformation (DSM), a very rare, congenital form of dural arteriovenous shunt (DAVS), typically affecting newborns. Methods Ultrasound (US) and MRI were performed at 34 weeks' gestation, and the findings of these examinations were compared with postnatal MRI studies performed at 2 days and 1 month. Results US showed an anechoic, midline posterior fossa collection with irregular internal echodensities. Color Doppler showed prominent arterial vascularity at the lesion margins. The prenatal MRI showed a large, profoundly hypointense, midline retrocerebellar mass. Postnatal MRI, complemented with magnetic resonance (MR) angiography, showed the lesion to be a giant dural venous pouch fed by multiple mural arteriovenous shunts. Follow-up MRI at 1 month suggested latent venous hypertension and prompted endovascular treatment. Conclusion Prenatal MR imaging is useful to establish the diagnosis, to assess complications such as hydrocephalus and tonsillar prolapse, and to help plan perinatal management, postnatal follow-up, and treatment decision-making. Copyright (c) 2006 John Wiley & Sons, Ltd.