Among the factors that determine the behavior of an intracranial aneurysm is the relationship between its volume and the size of the orifice. The investigative method described herein is the means being used to define that relationship in humans. It is a postmortem study that focuses on unruptured aneurysms. Central to the protocol was a synthetic rubber cast of the aneurysm's interior. The cast was made under normal arterial pressure so that unruptured aneurysms were reexpanded to lifelike size and shape. After the cast was removed intact from the specimen, the lumenal features recorded upon it were verified by comparison with the opened aneurysm. Working now only with the cast, the chamber was cut from the artery through its neck. The orifice area was determined by dividing it, mathematically, into many smaller, measurable forms. Chamber volume was ascertained by a fluid displacement technique. Both measurements were made with magnification and engineering instruments. Casts of ruptured or thrombosed aneurysms gave helpful morphological information, but were of limited value for measurement. The techniques are described, and examples of the initial results are presented.