Perfluorooctanoate and perfluorooctanesulfonate are used in many industrial products and have been widely detected in human blood. Both chemicals are associated with tumor development in animal studies, but data on carcinogenic potential in humans are sparse. We investigated the association between plasma levels of perfluorooctanoate and perfluorooctanesulfonate and cancer risk within a prospective Danish cohort of participants with no previous cancer diagnosis at enrollment. From enrollment, between December 1, 1993, and May 31, 1997, and through July 1, 2006, we identified 713 participants with prostate cancer, 332 with bladder cancer, 128 with pancreatic cancer, and 67 with liver cancer in the entire cohort and we selected a comparison subcohort of 772. Plasma concentrations of perfluorooctanoate and perfluorooctanesulfonate were measured in each participant by use of high-pressure liquid chromatography coupled to tandem mass spectrometry. We found no clear differences in incidence rate ratios for these cancers in relation to plasma concentrations of perfluorooctanoate or perfluorooctanesulfonate. A 30%-40% increase in risk estimates for prostate cancer was observed for the three upper quartiles of perfluorooctanesulfonate concentration compared with the lowest quartile (eg, for the lowest vs the fourth quartile, incidence rate ratio = 1.38, 95% confidence interval = 0.99 to 1.93). Plasma concentrations of perfluorooctanoate and perfluorooctanesulfonate in the general Danish population appear not to be associated with risk of prostate, bladder, pancreatic, or liver cancer.