This epidemiological study was based on a hematological and a molecular analysis of 310 heterozygous beta thalassemic and 75 carriers of Plb Lepore out of 3,000 microcythemic subjects from the Campania region of Italy. The molecular analysis of beta chains and the delta beta hybrid gene has shown different beta chain defects, but only the Hb Lepore-Boston-Washington type in association with haplotypes I and V. The prevalence and distribution of these molecular defects in Campania show that they are linked to historical events and to the geographical characteristics of this region.