This paper reviews the molecular pathology of a heterogeneous group of beta-thalassemia heterozygotes which may be referred to as atypical beta-thalassemia. This group includes four different categories of heterozygous beta-thalassemia, which are characterized, respectively, by (1) normal MCV and MCH; (2) normal Hb A2; (3) normal MCV, MCH, and Hb A2 and imbalanced globin chain synthesis only or, (4) the presence of clinical manifestations. The first group is represented by a limited proportion of double heterozygotes for alpha- and beta-thalassemia. The second group includes two categories. One category is double heterozygotes for delta- and beta-thalassemia with the delta-thalassemia mutation in cis or in trans to beta-thalassemia. A number of delta-thalassemia mutations which produce this phenotype by interacting with beta-thalassemia have been described. The other category within the second group is heterozygotes for some mild beta(+)-thalassemia mutations. Within the third group, conclusive evidence for a mutation within the beta-globin gene cluster producing the silent beta-thalassemia phenotype has been obtained solely for a C----T substitution at -101 within the CACCC box of the beta-globin gene. Possible candidates are the complex rearrangements (-T, +ATA; -T, +ATATA) found at position -530 from the cap site. In the group of thalassemic hemoglobinopathies, a series of mutations mostly located in the third exon and producing elongated or truncated molecules have been recently reported. Most of the mutations are silent at the protein level, produce inclusion bodies in peripheral erythrocytes, and show a dominant transmission pattern or occur sporadically.