Results of studies have shown that there is an excess risk of venous thromboembolism in users of oral contraceptives containing cyproterone compared with those containing levonorgestrel. We did a case-control study, in which we assessed the risk of idiopathic venous thromboembolism in women taking combined low-dose oestrogen oral contraceptives containing cyproterone (n=24401) or levonorgestrel (n=75000). We compared the 26 women in this population who had idiopathic venous thromboembolism with 144 matched controls. 12 individuals and 30 controls were taking contraceptives that contained cyproterone. Our results suggest that risk of venous thromboembolism is increased four-fold in women taking contraceptives containing cyproterone by comparison with those exposed to levonorgestrel.