The prevalence of anticardiolipin antibodies (ACLA) in sera of 49 patients having had their first TIA or ischemic stroke before 50 years of age was studied using a solid phase enzyme immunosorbent assay (ELISA). Five patients had IgM antibodies, eight had IgG, and three had antibodies belonging to both classes. Although ACLA were detected in 32% of patients (95% confidence interval [CI] 19-45%), the ACLA positive group did not differ with respect to clinical characteristics and distribution of major stroke risk factor frequency from the ACLA negative group. Further investigations are needed to establish the role of ACLA in the pathogenesis of ischemic cerebrovascular diseases.