The effects of variations in alpha-gene numbers on phenotypical expression of beta-thalassemia are assessed in 11 subjects of 8 families. The study indicates that coexistence of alpha-thalassemia (-alpha3.7/alpha alpha) decreases the HbF in IVSI-6 homozygote and in delta beta thalassemia trait and may ameliorate the disease in beta-thalassemia compound heterozygotes associated with one mild and one severe beta-thalassemia mutation. Coexistence of alpha-gene triplication is associated with an increase in HbF value and may increase the severity of beta-trait or beta-thalassemia intermedia. The effects of alpha-gene triplication on phenotypic expression of beta-thalassemia trait may not be uniformly observed in every subject affected with a similar genotype.