In 1979, there was an outbreak of food poisoning in central Taiwan due to cooking oil contaminated with polychlorinated biphenyls and their thermal degradation products. Starting in 1985, we studied 128 children born to exposed women after the oil was removed from the market; the exposure of these children was transplacental or through breast milk. We also studied matched controls. The exposed children exhibited developmental delays as measured by parental report, by neurologic examination, and by standard cognitive tests; delay was seen at all ages and persisted over time. Delay was greater in children who were smaller in size and in children who had exhibited neonatal symptoms of intoxication. Children with a history of nail deformity also were delayed. However, there was little relationship between other physical findings or measures of maternal exposure and developmental delay. There was some indication that the child's prenatal exposure was more important to developmental delay than was exposure through breast milk.