There is considerable interest in herbal therapies for cancer prevention but often with little scientific evidence to support their use. In this study, we examined epidemiological data regarding effects of commonly used herbal supplements on risk for ovarian cancer and sought supporting biological evidence. 4.2% of 721 controls compared to 1.6% of 668 cases regularly used Ginkgo biloba for an estimated relative risk (and 95% confidence interval) of 0.41 (0.20,0.84) (p=0.01); and the effect was most apparent in women with non-mucinous types of ovarian cancer, RR=0.33 (0.15,0.74) (p=0.007). In vitro experiments with normal and ovarian cancer cells showed that Ginkgo extract and its components, quercetin and ginkgolide A and B, have significant anti-proliferative effects ( approximately 40%) in serous ovarian cancer cells, but little effect in mucinous (RMUG-L) cells. For the ginkgolides, the inhibitory effect appeared to be cell cycle blockage at G0/G1 to S phase. This combined epidemiological and biological data provide supportive evidence for further studies of the chemopreventive or therapeutic effects of Ginkgo and ginkgolides on ovarian cancer.