Ticlopidine hydrochloride is an antiplatelet agent of proven antithrombotic efficacy that in December 1991 became available for general clinical use in the United States. The relative value of ticlopidine compared with aspirin, also an effective antiplatelet agent, has become a key clinical issue. Whereas ticlopidine is somewhat more effective than aspirin for preventing stroke in certain populations, it is also more expensive and potentially toxic. We recommend its use for patients with threatened stroke who are intolerant of aspirin and for patients who have cerebral ischemic symptoms despite aspirin therapy. Patients surviving major ischemic stroke make up a third group for whom ticlopidine use may be recommended in preference to aspirin. The use of ticlopidine rather than aspirin in patients with other cerebrovascular conditions is not strongly supported by existing data. The risk-benefit-cost equation involving ticlopidine versus other antithrombotic therapies is complex, rendering a wide range of acceptable management practices. If reliable laboratory monitoring for neutropenia during the first 3 months of therapy is not feasible, ticlopidine should not be used.