In this paper 33 cases of colloid cyst of the 3rd ventricle are presented. The initial symptoms as well as the clinical syndromes are quite variable, but usually consist of a combination of signs of increased intracranial pressure and an organic dementia, progressive or paroxysmal. Computed tomography is an invaluable diagnostic tool and it seems that, since it has become widely used in the evaluation of patients with headaches or dementia, colloid cysts are being recognized with increasing frequency. Different modalities of treatment are discussed. The transcortical and transcallosal approaches are both effective techniques, but the latter is probably preferable in cases with mild ventricular dilatation.