Turbulence within intracranial aneurysms may result in tearing of the aneurysmal wall, exposing the subendothelial matrix to circulating platelets. In this study, platelet interaction in giant intracranial aneurysms was evaluated by a dual-isotope technique employing In-labeled platelets and Tc-labeled red blood cells. The use of two isotopes allows the subtraction of the blood pool and the calculation of the ratio indium deposited:indium blood pool (In(D)/In(BP)). A ratio greater than zero indicates platelet deposition within aneurysm. Thirteen patients were evaluated in this way, with platelet deposition demonstrated in six. In these six patients, the ratio In(D)/In(BP) was found to be significantly elevated, with a mean value of 0.96 +/- 0.65. Three of these six patients has symptoms of recurrent transient neurological deficits; one of these three suffered a complete stroke following documentation of platelet deposition. In this case, the aneurysm was obtained at surgery and was found to contain intraluminal platelet aggregation when viewed by scanning electron microscopy. In the remaining seven patients, the ratio IN(D)/In(BP) was found not to be significantly elevated (mean -0.03 &/- 0.06), indicating the absence of active platelet deposition. Two of these patients had prior symptoms of cerebral ischemia; one of these was found to have an ulcer in the ipsilateral internal carotid artery which was probably responsible for thromboembolic events to the hemisphere. The authors conclude that platelet aggregation occurs more frequently than previously recognized in giant intracranial aneurysms, and their data substantiate the hypothesis that platelet metabolic products or thrombi originating from a large aneurysm may embolize to distal cerebral vessels.