To investigate the relation between low level lead absorption and mild mental retardation, hair lead concentrations were compared in a group of 40 mildly retarded children "etiology unknown" with a control group of 20 children. Children with probable cause for retardation were excluded from the sample as were children with a history of lead poisoning. The group of mentally retarded children "etiology unknown" had statistically significant raised hair lead concentrations. It was concluded that a reexamination of lead poisoning concentrations is needed, because concentrations of lead previously thought harmless may now have to be considered noxious and viewed as an etiological factor in mild mental retardation. (Author/SB)