Sonography and ultrafast magnetic resonance imaging were used in the prenatal assessment of two fetuses with sonographically suspected arachnoid cysts. In one case magnetic resonance imaging confirmed the ultrasound diagnosis, in the other case magnetic resonance imaging revealed the presence of a midline cyst associated with dysgenesis of the corpus callosum. Ultrafast magnetic resonance imaging is a useful supplementary imaging method in the evaluation of pregnancies complicated by fetal cerebral anomalies.