ABO blood group distribution was studied in 222 Controls and in 148 symptomatic and symptomless patients with chronic Chagas' disease with different anatomo-clinics forms (congestive heart failure, visceromegalias and sudden death). The X² was estimated from contingency table and by Woolfs method. The X² obtained by these two methods showed that the group O frequency was significantly less in the symptomatic chagasic group as a whole and in those with "megas" when compared with the controls. B blood group was observed to be more frequent in chagasics with sudden death but the X² was only significant when calculated for contingency tables. The apparent relative protection confered by the O blood group to chagasics with respect to its evolution to symptomatic forms of the disease, could be explained by cross antigenicity among T. cruzi populations and ABO blood group or by other factors influencing or being influenced by the blood group distribution.
Chagas' disease; Blood group; ABO blood group