Evidence is accumulating to suggest that higher exposure to solar ultraviolet radiation (UVR) is associated with decreased risk of internal cancers, but data for ovarian cancer are unclear. We aimed to examine the association between lifetime ambient UVR and ovarian cancer in a population-based-case-control study. The study included women aged 18 to 79 years with a new diagnosis of invasive (n = 1,215) or borderline (n = 285) epithelial ovarian cancer identified through a network of clinics and state cancer registries throughout Australia. Controls (n = 1,459), frequency matched to cases by age (5-year groups) and state of residence, were randomly selected from the National Electoral Roll. We asked participants to report where they had lived at different periods of their life and assigned an estimate of UVR using data from NASA's Total Ozone Mapping Spectrometer database (1997-2003). We estimated the association between ambient UVR and risk of ovarian cancer using conditional logistic regression adjusted for potential confounders. Women in the highest third of average daily ambient UVR over their lifetime were at significantly lower risk of all epithelial ovarian cancers than those in the lowest third [OR, 0.70; 95% confidence interval (CI), 0.56-0.88]. The inverse association was stronger for borderline tumors (0.47, 0.31-0.71) than invasive tumors (0.78, 0.61-1.00). The effect sizes for overall and borderline tumors were unchanged after adjusting for confounders, whereas the inverse association for invasive tumors was attenuated. These data suggest that exposure to ambient UVR may reduce the risk of ovarian cancer.