A thymidine deletion at position +10 of the 5' untranslated region of the beta-globin gene was detected in a beta-thalassaemia intermedia patient carrying a beta(0)39 stop codon mutation on the other chromosome; this new mutation, +10(-T), was detected by automated fluorescent DNA sequencing and verified by dot-blot allele-specific hybridizations. The +10(-T) mutation is a 'silent carrier', is associated with a reduced amount of steady-state beta-globin mRNA, and establishes a connection between the 5' untranslated region of the beta-globin gene and the regulation of its expression.