In the course of a prenatal diagnosis for beta thalassaemia by linkage analysis of restriction fragment length polymorphisms, a homozygous beta thalassaemia fetus was misdiagnosed as beta thalassaemia trait. Extensive studies of the polymorphic sites within the beta globin gene cluster in all the members of the family resulted in the conclusion that the paternal chromosome 11 of the newborn was different from that expected. Paternity was confirmed by HLA typing and blood group studies. The analysis of another polymorphic locus on chromosome 11 within the family was in agreement with the possibility of a crossing over between the two paternal chromosomes in a region 5' to the beta gene, previously indicated to contain a 'hot spot' area for recombination. This report underlines the risk of performing prenatal diagnosis using restriction polymorphisms 5' to the beta gene.