We have identified three individuals of Greek or Greek Cypriot origin with an atypical form of HbH disease characterized by a severe hypochromic microcytic anaemia associated with relatively small amounts of HbH in the peripheral blood. Molecular analysis has shown that each is a compound heterozygote for a previously described mutation affecting the poly A addition signal (AATAAA-->AATAAG) and a previously undescribed mutation involving a T-->C transition in codon 29 of the alpha 2 gene causing a leucine-->proline substitution. Although this mutation would be expected to produce an unstable haemoglobin and hence a haemolytic anaemia, simple heterozygotes for the alpha 29Leu-->Pro mutation have the phenotype of alpha-thalassaemia trait.