The ABO blood groups were determined in 125 patients suffering from venous thrombosis in a Brazilian population. There is a clear effect of the sex on the disease incidence, women being more frequently affected, but the mean age was not different regarding the sex. No differences were found in the disease incidence when Caucasians and Negroids were compared. An excess of blood group A and a decrease of blood group O was observed among the patients. The analysis of the combined data from ten different published series shows a A/O relative incidence significantly higher than unity, but the heterogeneity was also significant in the series.