Severe Hb H disease presented in unexpected ways in 2 families of Greek origin. In 1, Hb H disease led to neonatal death. The underlying molecular defect was double-heterozygosity for the --Med/ alpha thalassaemia haplotype and a nondeletional alpha thalassaemia defect (alpha alpha T'Karditsa'/). The 2nd family requested antenatal diagnosis. The husband had mild nondeletional alpha thalassaemia. Initial investigations in the wife demonstrated unexpected gene mapping patterns. These have recently been shown to result from the (-alpha)Med 20.5/ haplotype.