Colloid cyst of the third ventricle is a rare, benign pathology of endothelial origin. Excellent prognosis is achieved when the cyst is diagnosed early and excised. Hydrocephalus can occur as an acute, fatal complication or can cause chronic symptoms such as headaches. A case of a patient with a third ventricle colloid cyst in association with prepontine subarachnoid haemorrhage is presented. This was a 29-year-old healthy male who was admitted because of a depressed consciousness. Neuro-radiological imaging revealed a mass in the third ventricle, a biventricular hydrocephalus and a prepontine subarachnoid haemorrhage without any evidence of a cerebral aneurysm. Perimesencephalic nonaneurysmal subarachnoid haemorrhage is well known but its cause is still a matter of debate. A description is given of the association of this type of subarachnoid haemorrhage with an acute hydrocephalus caused by a third ventricle colloid cyst. Because of the potential morbidity and mortality of third ventricle colloid cysts, this association is described and should be considered as a rare presentation of third ventricle colloid cyst.