Thirty-six colloid cysts were treated from 1949 to 1983. There were 26 male and 10 female patients, ranging in age from 12 to 65 years old, 60% between 31 and 40 years. Headache or disturbed mental function was the most frequent complaint, papilloedema the most frequent sign. The patients were classified into 3 groups by symptoms and signs; Group I (17 patients): Headache, papilloedema and no neurological signs. Group II (6 patients): Fluctuating or progressive dementia. Group III: Twelve cases with "classical" features, episodic headache and drop attacks. One patient could not be classified in any of these groups. Seventeen of 36 patients were diagnosed by ventriculography, 19 patients were diagnosed by CT scan. CT scan was the most reliable diagnostic study, but was unavailable in the earlier part of the series. All patients have been operated by transventricular exposure of the right foramen of Munro with incision of the middle frontal gyrus in an antero-posterior linear manner. Twenty-five of 36 patients showed an excellent operative result, and nine had a good result, one of the 9 patients dying of an unrelated intracerebral haemorrhage 4 years after operation. Two patients had a poor result, characterized by memory loss and confusion. One of these died of an unknown cause 5 years after operation.