OBJECTIVE--To determine whether the distribution of ABO blood groups in women with ovarian cancer differs from that in the general population in a large, defined English region. DESIGN--Analysis of record abstracts of hospital care held in the Oxford record linkage study supplemented with data from the Oxford cancer registry. SETTING--Oxford Regional Health Authority area. SUBJECTS--A total of 1261 women who had ovarian cancer between 1968 and 1986 with ABO blood groups recorded on the Oxford Record Linkage Study and cross checked against the cancer registry comprised the study group. MEASUREMENTS AND MAIN RESULTS--The relative incidence of A:O and B:O blood groups in women with ovarian cancer were compared with the general population in the same region. Ovarian cancer was more common in women of blood group A than in others, with a relative incidence of 1.17. In particular, adenocarcinomas were the most common type of tumour and were associated with blood group A. The association was more striking in married women than in single women probably reflecting differences associated with parity. CONCLUSION--The association between ABO blood groups and ovarian cancer found in this English population is similar in size to that reported from several other populations. Childbearing is known to reduce the risk of ovarian cancer and our findings suggest that the blood group association may be most apparent in married, parous (that is, relatively low risk) women.