The known relationship between the retardation of polarized light and the molecular organization of collagen enabled us to study collagen in cerebral saccular aneurysms, structures in which the collagen is probably abnormal. Six aneurysms and their adjacent arteries, obtained at autopsy, were sectioned at a thickness of 7 microns. Measurements of retardation in hematoxylin and eosin stained sections, obtained by the method of de Sénarmont, were significantly lower in the aneurysm wall compared with the adventitial region of adjacent arteries. A similar reduction in retardation was found by imbibition analysis on unstained sections from three of the aneurysms. We propose that the difference in optical properties arises because the aneurysm collagen is immature and so is less organized at the molecular level. The presence of immature collagen could also explain the altered mechanical properties that are necessary for the enlargement and rupture of saccular aneurysms.