We report a case in which the interaction of heterozygosis for both the beta(0)-IVS-II-1 (G-->A) mutation and the alphaalphaalpha(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 beta-thalassemia trait. Investigation of the alpha-globin gene status revealed heterozygosis for alpha-gene triplication (alphaalphaalpha/alphaalpha). The patient's father, also presenting mild microcytic and hypochromic anemia, had the same alpha and beta genotypes as his son, while the mother, not related to the father and hematologically normal, was also a carrier of the alphaalphaalpha(anti-3.7) allele. The present case emphasizes the need for considering the possibility of alpha-gene triplication in beta-thalassemia heterozygotes who display an unexpected severe phenotype. The beta-thalassemia mutation found here is being described for the first time in Brazil.366699701