The authors describe their first clinical experiences in endoscopic third ventriculostomy (ETV) with the original ultrasonic contact microprobe (UCM) designed at the Department of Neurosurgery in Zagreb. The analysis includes the clinical course of disease in eight patients submitted to surgery from May to September 1999 (3 men and 5 women, from 14 to 61 years of age). Surgery was performed in patients with neurological symptoms of elevated intracranial pressure and neuroradiological evidence of non-communicating hydrocephalus caused by mesencephalic aqueduct stenosis. The perforation in the base of the third brain ventricle made by the ultrasonic contact microprobe was widened by a balloon catheter. The authors have come to conclusion that the ETV when performed by contact ultrasonic microprobe is a small risk procedure in case of non-communicating hydrocephalus. For its small diameter (1.6 mm) and simple handling the newly designed contact ultrasonic microprobe is very suitable for use in neuroendoscopy as it enables fenestration of the third brain ventricle with minimal thermal and ultrastructural damage to the adjacent neurovascular structures. Further research will be focused on defining indications for the use of the device in other neuroendoscopic procedures as well.