Cerebral ventricular dilation and/or frank hydrocephalus unrelated to brain tumors is not a rare feature of von Recklinghausen neurofibromatosis (NF-1). In our experience, it was observed in 23% of patients consulting for neurological problems. From 1984 to 1989, 30 patients with documented NF-1 were referred to for cranial or spinal magnetic resonance imaging (MRI). In 9 of these patients, clinical features or previous computed tomographic scanning revealed hydrocephalus or ventriculomegaly. One patient had biventricular hydrocephalus (enlarged lateral ventricles), 7 had triventricular hydrocephalus (aqueductal stenosis) and 1 had tetraventricular hydrocephalus (obstruction of the foramina of Magendie and Luschka). MRI showed anomalies in all 9 patients. The importance of MRI for the diagnosis, control and treatment of the hydrocephalus as well as for understanding the embryologic pathogenesis of these conditions is emphasized.