Colloid cysts of the 3rd ventricle, diagnosis and treatment. 11 patients with colloid cyst of the 3rd ventricle were diagnosed and treated at the Department of Neurosurgery the Medical University of Warsaw, between 1987 and 1996. The patients comprised 5 females and 6 males, aged 22 to 58 years. Mean time from the occurrence of the first symptoms to surgery was 23 months, although this varied in individual cases from 8 days to 10 years. In the majority of patients, the leading symptoms were: headaches, nausea, vomiting and papilloedema, comprising symptoms of high intracranial pressure. Slowly progressing symptoms of organic brain damage syndrome were observed in 5 patients. Preoperative diagnosis was based on CT of the head in 8 patients, on NMR in 2 and on both CT and NMR in 1 patient. At the first stage of treatment, a ventriculo- peritoneal Y-shunt was used in 3 patients to eliminate symptoms of high intracranial pressure. All patients then underwent radical surgery. 9 patients were treated by craniotomy in the frontal region with incision of the cortex and then through the lateral ventricle and interventricular foramen. 1 patient was operated by craniotomy and through the terminal lamina of the 3rd ventricle. In 1 case the cyst was evacuated via neuroendoscope. There were no cases of mortality among our patients. Full recovery occurred in 8 patients. In 3 patients symptoms of organic brain damage syndrome observed prior to surgery remained, but in a lesser degree, which may be a result of the correct diagnosis being established too late. In over 5 years' follow-up, no cases of recurrence were observed.