To investigate possible relationships between polychlorinated biphenyl (PCB) exposure and infectious disease mortality in harbor porpoises (Phocoena phocoena) in United Kingdom waters, summed blubber concentrations of 25 chlorobiphenyl congeners (Σ25CB) in healthy harbor porpoises that died of acute physical trauma (mainly by-catch; n = 175) were compared with Σ25CB values in animals that died of infectious disease (n = 82). The infectious disease group had significantly greater Σ25CB values (mean, 27.6 mg/kg lipid) than the physical trauma group (mean, 13.6 mg/kg lipid;p < 0.001). This association occurred independently of other potentially confounding variables, including age, sex, two indices of nutritional status, season, region, and year found. Total blubber PCB levels (as Aroclor 1254) were also calculated, enabling direct comparison with a proposed threshold for adverse health effects (including immunosuppression) in marine mammals of 17 mg/kg lipid. In porpoises with total PCB levels greater than 17 mg/kg lipid (n = 154), total PCB levels were significantly higher in the infectious disease group compared to the physical trauma group (p < 0.001). This association was no longer significant in porpoises with total PCB levels of less than 17 mg/kg lipid (n = 103; p > 0.55). These findings are consistent with a causal (immunotoxic) relationship between PCB exposure and infectious disease mortality, and they provide a framework for future quantitative risk-assessment analyses of porpoise populations of known size and PCB exposure.