We have been following up the biological and mental development of children exposed prenatally to polychlorinated biphenyls and their contaminants (Yucheng children). When we started this 12-year follow-up study in August 1985, 118 Yucheng children we assigned a non-exposed child matched by sex, age, locality of residence, mother's age, socio-economic status of the family. This article reports the cognitive aspect of the development of Yucheng children as compared to their matched controls. A consistent tendency which indicates that Yucheng children score lower in each kind of measurement tool at each age level has been observed. This seems to imply that congenitally exposure to PCBs and their contaminants has long-term adverse effects on the cognitive development of human being.