Polychlorinated biphenyls (PCBs) suppress immunity, and may have exacerbated the effects of recent morbillivirus epizootics in harbour seals (Phoca vitulina) and striped dolphins (Stenella coeruleoalba). To test the hypothesis that PCBs cause immunosuppression in harbour porpoises (Phocoena phocoena) and increase their risk of dying from an infectious or parasitic disease, we compared the levels of PCBs in the blubber between animals that died from these causes and those that died from physical trauma. We established the cause of death and measured the blubber levels of HCB, α-HCH, γ-HCH, p,p′-DDE, p,p′-DDT, p,p′-TDE, dieldrin, and 25 individual chlorobiphenyls in 94 harbour porpoise carcases, found in Great Britain between 1989 and 1992. After correcting for the effect of region, there were no significant differences between disease groups in the levels of any of these contaminants. Thus, our results provide no support for the hypothesis. However, there were significant differences in body condition between animals that died from an infectious or parasitic disease, physical trauma, or starvation. Therefore, we suggest that a quantitative measure of body condition, such as relative body girth, could be used as a diagnostic aid in the pathological examination of harbour porpoises.