Lead-induced toxicity varies among individuals partly because of genetic differences in their susceptibility to the metal's effects. One genetic polymorphism associated with lead toxicity is a G-to-C transversion at position 177 in the coding region of delta-aminolevulinic acid dehydratase (ALAD), originating from two codominant alleles (ALAD1 and ALAD2). We examined the distribution of this single nucleotide polymorphism in two populations from the Iberian Peninsula. Genomic DNA was extracted from whole blood, and a real-time PCR assay was designed to determine ALAD polymorphic distribution. The distribution of MspI polymorphism was similar in the two populations studied, and allelic frequencies were comparable to those obtained in other studies of Caucasians. Further studies are needed to assess fully the functional significance of this polymorphism and its influence on the toxicokinetics of lead.