We report a case in which the interaction of heterozygosis for both the 0-IVS-II-1 (G->A) mutation and the alpha alpha alpha anti-3,7 allele was the probable cause for the clinical occurrence of thalassemia intermedia. The propositus, a 6-year-old Caucasian Brazilian boy of Portuguese descent, showed a moderately severe chronic anemia in spite of having the -thalassemia trait. Investigation of the alpha-globin gene status revealed heterozygosis for alpha-gene triplication (alpha alpha alpha / alpha alpha). The patient's father, also presenting mild microcytic and hypochromic anemia, had the same alpha and genotypes as his son, while the mother, not related to the father and hematologically normal, was also a carrier of the alpha alpha alpha anti-3,7 allele. The present case emphasizes the need for considering the possibility of alpha-gene triplication in -thalassemia heterozygotes who display an unexpected severe phenotype. The -thalassemia mutation found here is being described for the first time in Brazil.