Release of perfluorooctanoic acid (PFOA) from a chemical plant in West Virginia contaminated drinking water in several water districts, but use of granular activated-carbon filtration in two water districts reduced PFOA concentrations to levels below the limit of detection. Bartell et al. (p. 222) measured serum PFOA concentrations in up to six blood samples collected over approximately 12 months from a stratified random sample of 200 eligible participants in the C8 Health Project cohort, including baseline samples collected before initiation of water filtration to remove PFOA. The authors report that the average covariate-adjusted average rate of decrease in serum PFOA concentration after water filtration was 26% per year [95% confidence interval (CI), 2528% per year], consistent with a median serum PFOA half-life of 2.3 years (95% CI, 2.12.4 years). However, the authors also noted substantial variation in estimated half-lives among individuals, which may reflect ongoing exposures among some participants (e.g., through consumption of contaminated home-grown produce) and/or individual differences in PFOA elimination rates. In addition, they suggest that the accuracy of half-life estimates will improve with longer follow-up.