Considerable evidence from the United Kingdom and North America has shown that oral contraceptive use is associated with an increased risk of cardiovascular disease. However, since little is known about the link between the two in other parts of the world, WHO initiated a hospital-based case-control study in three centres in Mexico, Hong Kong, and the German Democratic Republic. Both cases and controls were asked about their past and current use of contraception as well as details on a number of potential confounding factors. Three main diagnostic categories were studied: venous thromboembolism (VTE) and pulmonary embolism (PE), ischaemic heart disease (IHD), and stroke. An overall relative risk of 2.9 (95% confidence limits, 1.4-6.1) for VTE/PE was found among recent or current users of oral contraceptives. Although this elevated risk is consistent with the results of other studies, the possibility of detection bias cannot be ruled out. The small number of cases (8) of IHD identified in the course of this study greatly limited the conclusions that could be drawn for this disease. Similarly with stroke, the small number of cases limited the conclusions that could be drawn, particularly since it was not possible to distinguish between thrombotic and haemorrhagic stroke. In addition to suggesting an increased risk of VTE/PE, the study pointed out the importance of ensuring an adequate sample size based on newly-diagnosed cases, the need for a coordinating centre to monitor the study closely in each centre and to provide a central review of each case, and the necessity of more specific diagnoses for meaningful interpretation of the data.