The significance and cause of ventriculomegaly in achondroplasia was investigated in five achondroplastic children. The intraventricular pressure (IVP) was monitored over 24 hours, followed by intraventricular injection of radionuclide alone or in combination with water-soluble contrast material. The IVP was elevated and the reabsorption of cerebrospinal fluid (CSF) into the sagittal sinus was slow in all cases, but there was no obstruction to CSF flow. The spinal subarachnoid space was well seen in all patients. Jugular venograms with pressure monitoring were obtained in four patients (bilaterally in one). These studies confirmed a narrow jugular foramen in all patients with a significant venous pressure gradient (3 to 10 mm Hg) obtained while the catheter was being pulled back from the sigmoid sinus through the foramen. A second gradient was found in the jugular vein in two patients at the level of the upper thoracic aperture. This gradient was 6 and 14 mm Hg, respectively. Identical venograms and monitoring of the venous pressure in a control group showed no pressure gradients across the jugular foramen and smaller gradients (2 to 5 mm Hg) across the thoracic inlet. It is concluded from these studies that ventriculomegaly in achondroplastic children represents hydrocephalus, which is likely secondary to raised intracranial venous pressure due to hemodynamically significant stenosis of the jugular foramen and, in some cases, the jugular vein in the thoracic aperture.