We describe a family with beta thalassaemia, apparently not linked to the beta-globin gene cluster, in combination with alpha thalassaemia. The propositus, an adult Dutch Caucasian male, and his son presented with microcytic hypochromic parameters. Their lysates displayed the normal adult pattern on electrophoresis. The HbA2 concentration, which is usually increased in beta thalassaemia, was normal. The in vitro biosynthetic rate of the globin chains was strongly unbalanced even in the presence of a coexisting alpha-thalassaemia defect. Routine analysis of the beta genes, including the promoter region, was performed repeatedly by polymerase chain reaction (PCR), denaturing gradient gel electrophoresis (DGGE) and direct sequencing. No molecular abnormalities were detected. Large beta deletions were excluded by haplotype determination, using seven polymorphic markers distributed over an area of 50 kb, from 1 kb 5' of the epsilon gene to 4 kb 3' of the beta gene. The haplotype analysis of the beta-gene cluster revealed that the unaffected daughter had received the same beta haplotype as her beta-thalassaemic brother from their beta-thalassaemic father. These data suggest that the beta-gene cluster shared by father and son was not directly associated with a reduced beta-globin chain expression. In order to exclude the remote possibility of a beta-locus-control region (LCR) rearrangement in the paternal haplotype of the daughter, the sequence of the HS2 element was examined in the nuclear family. We compared the haematological and clinical data of this family with the data reported in the limited number of similar cases. We discuss the possibility that the mutation of a trans-acting erythroid factor(s), not linked to the beta-genes cluster, may impair the beta-gene expression of both alleles.