Potts: There are differences in the risks associated with hormonal contraception in relation to smoking, obesity and previous patterns of thromboembolic disease. In a study carried out in Bangkok and Oxford (Chumnijarakij & Poshyachinda, 1975) comparing the incidence of thrombosis, as measured by 125I-labelled fibrinogen, after hysterectomy or cancer surgery, there was a higher incidence of postoperative thrombosis, measured by objective criteria, in Oxford (12–35%) than in Bangkok (1·7–3·8%). The argument could be made by inference that Asian women have less risks with the pill, although there are no data to prove it either way.