The investigators tested the hypotheses that young children with SCA experienced a variable period of asymptomatic progressive central nervous system (CNS) vasculopathy prior to cerebral infarction; that pre-infarct CNS vasculopathy could be identified by non-invasive imaging techniques:MRI, magnetic resonance angiography (MRA), and transcranial Doppler (TCD); and that therapeutic intervention at this stage of the disease could significantly reduce the subsequent occurrence of cerebral infarction. MRI, MRA, TCD, and standardized neurologic and psychometric examinations were performed yearly in a cohort of homozygous Hb SS children enrolled at 2-4 years of age. Subjects without MRI evidence of cerebral infarction who had significant cerebral vasculopathy (cerebral arterial stenosis on MRA and/or elevated blood flow velocity on TCD) were randomized to receive either no therapy or chronic transfusion therapy, in order to determine the risk of subsequent cerebral infarction in untreated subjects with these abnormalities, and the extent to which transfusion therapy could significantly reduce the risk. Subjects with evidence of prior cerebral infarction on MRI, whether symptomatic or asymptomatic, were randomized to receive either chronic transfusion therapy alone ('standard therapy') or chronic transfusion therapy plus ticlopidine, in order to determine whether ticlopidine could significantly increase the efficacy of standard therapy in preventing recurrent cerebral infarction in SCA. Subjects with prior cerebral infarction were also offered the option of bone marrow transplantation if an HLA-identical non-SS sibling donor was available.