An interinstitutional cooperative study in determining the effectiveness of the oral administration of acetylsalicylic acid (aspirin) in the reduction or prevention of transient cerebral ischemic attacks and amaurosis fugax. In addition, the effect on the frequency of cerebral and myocardial infarction in long-term follow-up will be evaluated. This is a controlled clinical trial in which only individuals having the hemispheric type of transient ischemic attack or monocular blindness will be included. Those with primary vertebrobasilar insufficiency only, or with potentially fatal complicating medical conditions, will be excluded. Care will be taken to exclude individuals found to have blood dyscrasia or specific idiosyncrasy to aspirin. Subjects will be allocated to subgroups on the basis of a clinical decision to operate or not to operate. In each of the two subgroups, subjects will then be assigned to treatment with aspirin or a placebo on the basis of a random statistical design. This design will enable the investigators to make meaningful comparisons between therapies in each subgroup. Strict surveillance will be maintained of the patient's adherence to the insturctions for therapy. The administration of the active drug and placebo will be on a coded, double blind basis. A mechanism is provided for in the project design for breaking the code in the event of a clinical necessity. The facilities to be used for data collection storage, retrieval and analysis will be those used for the Central Registry of the Joint Study of Extracranial Arterial Occlusion, supported by the National Heart and Lung Institute.