The association between ABO blood groups and Achilles tendon (AT) ruptures was studied in 215 consecutive AT rupture patients treated at Oulu University Hospital during the 16-year period from 1979 to 1994 as compared with control material consisting of earlier blood group determinations performed on an unselected sample of 5,536 young Finnish male adults. There was no blood group O dominance or other statistical differences in ABO blood groups between the patients with AT rupture and the control population (chi 2 3.79, P = 0.28), the A/O ratio being 1.82 in the rupture group and 1.42 in the controls. We found no blood group O dominance in competitive athletes, recreational athletes or non-athletes, in patients with sports-related AT ruptures or non-sports-related ruptures and in younger (< 45 years) or older (> or = 45--years) patients. In conclusion, our results do not confirm early findings of blood group O dominance in patients with AT rupture.