The diagnosis of colloid cyst of the third ventricle is notoriously difficult and contributes significantly to the high mortality. Three cases are presented in which computed tomography has contributed to the diagnosis by showing (1) the dilatation of the lateral ventricles without dilatation of the midline ventricular system, and (2) the lesion as an area of increased density within the third ventricle. Computed tomography may reduce the overall mortality in this condition by reducing the number of patients who succumb undiagnosed during an acute phase of raised intracranial pressure.