Only few reports can be found on endoscopic third ventriculostomy (ETV) in the Polish literature, and the majority of other reports concern paediatric or mixed population. This has induced the authors to report their experience with ETV in adults, reporting the results and discussing the usefulness and effectiveness of this procedure, causes of complications and failure. ETV was carried out in 20 patients aged over 18 years in a two-year period, beginning in 1999. In 13 cases (64%) the cause was external compression of CSF system by tumour leading to hydrocephalus. In 3 cases aqueduct stenosis was producing hydrocephalus, in 3 cases arachnoid cyst, perisellar or situated in posterior part of the third ventricle, was the cause, and in one case colloidal cyst of the third ventricle. The outcome were analysed according to clinical and radiological criteria finding that the ETV was successful in 90% of cases by clinical criteria, and in 88% by radiological criteria. Only unimportant clinical complications were reported without major consequences. It is concluded that ETV is a very useful method for hydrocephalus treatment in adults, especially if caused by blockade of CSF pathways by tumour or arachnoid cysts in the vicinity of the third ventricle.