Despite the enormous number of reports on polychlorinated biphenyl (PCB) toxicology, both the causal interpretation of epidemiological studies and the risk assessment of human exposures have been hampered by the lack of information on the pharmacokinetics of various PCB isomers and congeners. Thus, the assessment of exposure by means of measuring either total PCBs or individual congeners in the blood has so far been unsatisfactory. For example, the concentration and the pattern of congeners in the blood did not correlate with that at site(s) of action. In fact, the same levels of blood PCBs correlated with either toxic effects or no effects (both in clinical and epidemiological studies). In addition, when toxicity caused by PCBs was observed, the severity of the signs did not correlate with blood levels.Reasons for such a qualified failure are manifold and include different ways of reporting blood measurements, the different toxicological characteristics of each PCB, and different timing of sampling the blood, etc. Therefore, only limited conclusions can be drawn concerning what blood PCB measurements mean.