p-Dichlorobenzene is used widely in the United States as a room deodorizer, a moth repellent, and a precursor for a polymer. In a previous study of selected children in Arkansas, we found that 96% of the children had detectable urinary concentrations of 2,5-dichlorophenol, the metabolite of p-dichlorobenzene. In the current study, we found that, in a sample of 1,000 adults who lived throughout the United States, 98% had detectable levels of 2,5-dichlorophenol in their urine, and 96% had detectable levels of p-dichlorobenzene in their blood. Urinary 2,5-dichlorophenol concentrations ranged up to 8,700 micrograms/l (median and mean concentrations of 30 micrograms/l and 200 micrograms/l, respectively). p-Dichlorobenzene blood concentrations ranged up to 49 micrograms/l, with median and mean concentrations of 0.33 micrograms/l and 2.1 micrograms/l, respectively. The Pearson correlation coefficient for 2,5-dichlorophenol in urine and p-dichlorobenzene in blood was .82 (p < .0001), thus demonstrating a strong association between these exposure measurements. Neither age nor gender was related to urinary 2,5-dichlorophenol or blood p-dichlorobenzene concentrations (p > .40). When these results are viewed with data from other studies, the collective data show that p-dichlorobenzene is a common, worldwide contaminant. The high prevalence of exposure to p-dichlorobenzene, coupled with its potential for adverse health effects, indicate the need for more detailed studies, including studies of long-term health effects on exposed populations.