Adverse health effects following prenatal exposures to methylmercury (MeHg) have been apparent from several prospective cohort studies conducted in a fish-eating population. A prospective study in a Faroese birth cohort documented subtle deficits of several functional domains at prenatal MeHg exposure levels previously thought to be safe. Recent additional studies also showed neurobehavioral deficits associated with exposures to polychlorinated biphenyls (PCBs) with concomitant MeHg poisoning. In contrast, a prospective study in the Seychelles did not detect a similar association between MeHg exposure and neurodevelopmental deficits; children of the highest MeHg exposure group showed better scores in some developmental tests than those of the lower exposure groups for both prenatal and postnatal MeHg exposures. This paradoxical difference between both studies is summarized herein. The primary source of human exposure to MeHg is fish. Since a considerable number of pollutants, including polychlorinated biphenyls (PCBs) and pesticides, are also present in fish, and since some organochemical substances including PCBs are also well documented to be neurotoxic to the developing brain from epidemiological studies, the combined effects of these pollutants should be considered in discussing the neurotoxicity of MeHg. In this article, therefore, major prospective cohort studies focusing on the exposures to PCBs were reviewed.