Platelet membrane glycoprotein (GP) Ibalpha is a critical component of platelet adhesion complex to subendothelium structures following tissue injury or pathological surfaces, such as atherosclerotic plaques. Polymorphisms of the GPIbalpha gene have been associated with a high risk for occlusive vascular disease, and its distribution varies considerably among distinct populations. These polymorphisms comprise the human platelet antigen (HPA)-2 system, the -5C/T dimorphism of the Kozak sequence, and the variable number of tandem 39-bp repeats (VNTR). Here we report the prevalence of the GPIba gene polymorphisms among Brazilians, a highly ethnically diverse population. We analyzed 492 subjects of European, African, or Indigenous origin. It was possible to determine ten distinct haplotypes. The most common (similar to40%) haplotype was the Kozak-TT/HPA-2aa/VNTR-CC for both Caucasian and African descent. However, among Indigenous, Kozak-TT/HPA-2aa/VNTR-CC and Kozak-TC/HPA-2aa/VNTR-CC were equally present. Although a strong linkage disequilibrium between VNTR and HPA-2 polymorphism had also been observed, here we determined incomplete linkage disequilibrium in 10% of subjects from all ethnic groups. VNTR-E, a rare variant lacking the 39-bp repeat, was identified in two unrelated subjects, and functional platelet studies revealed no abnormalities. The VNTR-A allele, the largest variant containing four copies of the repeats, was not identified in this population. However, homozygosity for the VNTR-A allele (Kozak-TT/HPA-2aa/VNTR-AA) was determined in two distinct species of nonhuman primates. These results suggest a greater complex evolutionary mechanism in the macroglycoprotein region of the GPIbalpha gene and may be useful in the design of gene-disease association studies for vascular disease. (C) 2004 Wiley-Liss, Inc.772107116