Achondroplasts frequently have large heads, and even mild dilatation of the ventricles, but in most instances this is not due to hydrocephalus. For the diagnosis of hydrocephalus to be made in a patient with achondroplasia, an excessive rate of growth of the head must be established, as well as enlarged ventricles. Two achondroplastic children who developed hydrocephalus are described. The hydrocephalus was probably obstructive in one case, but not in the other. The hydrocephalus is thought to be related to the cranial abnormalities occurring in achondroplasia — shortened clivus, small foramen magnum, basilar impression — with resultant absence of the cisterna magna, although the mechanism is not clearly established. Another interesting finding was high termination of the basilar artery. Zwei achondroplastische Kinder werden beschrieben, welche einen Hydrocephalus entwickelten. In einem Fall handelte es sich um einen Verschluß-Hydrocephalus wahrscheinlich infolge der knöchernen Anomalien an der Schädelbasis. Im anderen Fall konnte aber kein Verschluß nachgewiesen werden.