53 per cent of the 873 cases of subcutaneous (spontaneous) tendon ruptures, treated in the National Institute of Traumatology belonged to the blood group O, and that was significantly higher than in the healthy population (31.1 per cent) of Hungary. The ratio of the tendon rupture patients belonging to the blood group A was significantly lower (27.1 per cent) than in the population (42,4 per cent). The ratio of the cases belonging to the blood group O among the multiple tendon rupture and rerupture patients proved to be over 70 per cent. On the basis of their data authors think that there is a relation between the ABO blood group system and the tendon rupture: those belonging to the blood group O are more endangered by, those belonging to the blood group A are more protected against tendon rupture. No relation was found between the Rh(D) blood group system and the tendon rupture.