The delta-aminolevulinate dehydrase (ALAD) genetic polymorphism was investigated along with biomarkers of lead exposure and effect on 333 male workers, occupationally exposed to lead, with blood lead levels (PbB) higher than 100 microg/l. ALAD genotype frequencies were distributed as expected among Caucasians. Workers bearing at least one ALAD-2 allele showed PbB values slightly higher than those from ALAD-1-1 subjects, who also exhibited higher urinary delta-aminolevulinic acid (ALAU) and blood zinc protoporphyrin (ZPP) levels. The plasmatic lead (PbP)/PbB ratio was similar in both groups. Exposure and effect biomarkers were significantly each other correlated among ALAD-1-1 subjects only, who showed also a significant inverse relationship between ALAD activity and ZPP values. Results confirm previous studies, supporting the hypothesis that ALAD polymorphism may interfere with toxico-kinetic and toxico-dynamic parameters of occupational exposure to Pb.