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The aim of this study is to describe the distribution of the platelet blood group A antigenicity in Euro-Brazilians (EUBs) and Afro-Brazilians (AFBs). A small but significant proportion of individuals express high levels of A or B antigen on their platelets corresponding to the erythrocyte ABO group. The mechanism of increased antigen expression has not been elucidated. A cohort of 241 blood group A donors was analysed by flow cytometry. Although mean fluorescence intensity (MFI) is a typical continuous variable, platelets were screened and divided into two categories: low expressers (LEs) and high expressers (HEs). A three-generation family was investigated looking for an inheritance mechanism. The prevalence of the HE platelet phenotype among group A donors was 2%. The mean of MFI on platelets of A subgroup of EUBs differs from that of AFBs ( P = 0·0115), whereas the frequency of the HE phenotype was similar between them ( P = 0·5251). A significant difference was found between sexes ( P = 0·0039). Whereas the serum glycosyltransferase from HE family members converted significantly more H antigen on group O erythrocytes into A antigens compared with that in LE serum, their ABO, FUT1 and FUT2 genes were consensus. The theoretically favourable, transcriptionally four-repeat ABO enhancer was not observed. The occurrence of HE in several members suggests familial aggregation. Indeed, in repeated measures, stability of the MFI values is suggesting an inherited condition. Factors outside the ABO locus might be responsible for the HE phenotype. Whether the real mechanism of inheritance is either of a polygenic or of a discrete Mendelian nature remains to be elucidated. [ABSTRACT FROM AUTHOR]