Six operatively treated patients are described who had sustained post-traumatic bleeds into a porencephalic cyst. The plain films of the skull showed asymmetries of the sphenoidal wings, bulging of the middle temporal fossae, circumscribed protrusions in the temporal region and unilateral enlargement of the paranasal sinuses. These changes are pathognomonic of a cystic malformation. Angiography shows an almost vertical origin of the middle group of vessels and a paucity of vessels in the area of the cyst. An accurate radiological diagnosis can be made pre-operatively, subdural haematomas and hygromas, arachnoid cysts and tumours entering into the differential diagnosis. Clinical evidence of raised intracranial pressure is an indication for performing burr holes; a porencephalic malformation does not in itself require surgical treatment.