Idiopathic stenosis of the foramina of Magendie and Luschka is a rare cause of obstructive hydrocephalus involving the four ventricles. Like other causes of noncommunicating hydrocephalus, it can be treated with endoscopic third ventriculostomy (ETV). Three patients who were 21, 53, and 68 years of age presented with either headaches (isolated or associated with raised intracranial pressure) or vertigo, or a combination of gait disorders, sphincter disorders, and disorders of higher functions. In each case, magnetic resonance (MR) imaging demonstrated hydrocephalus involving the four ventricles (mean transverse diameter of third ventricle 14.15 mm; mean sagittal diameter of fourth ventricle 23.13 mm; and mean ventricular volume 123.92 ml) with no signs of a Chiari Type I malformation (normal posterior fossa dimensions, no herniation of cerebellar tonsils). The diagnosis of obstruction was confirmed using ventriculography (in two patients) and/or MR flow images (in two patients). All patients presented with marked dilation of the foramen of Luschka that herniated into the cisterna pontis. All patients were treated using ETV. No complications were observed. All three patients became asymptomatic during the weeks following the surgical procedure and remained stable at a mean follow-up interval of 36 months. Postoperative MR images demonstrated regression of the hydrocephalus (mean transverse diameter of third ventricle 7.01 mm; mean sagittal diameter of fourth ventricle 16.6 mm; and mean ventricular volume 79.95 ml), resolution of dilation of the foramen of Luschka, and good patency of the ventriculostomy (flow sequences). These results confirm the existence of hydrocephalus caused by idiopathic fourth ventricle outflow obstruction without an associated Chiari Type I malformation, and the efficacy of ETV for this rare indication.