The clinical and haematological data of nine patients with haemoglobin H disease, as well as of 16 members of their families, are reported. Pedigree study and haematological data permit the assumption that in an individual four genes could be responsible for α-chain synthesis. Based on this hypothesis, four variants of α-thalassaemia are expected to exist. The first, in which only one gene is affected has no, or only minor, red cell changes and an increase in Hb Bart’s at birth, the second with two genes affected has in addition Hb H inclusion bodies in a few red cells, the third, with three genes affected represents haemoglobin H disease, and the fourth, with all four genes affected, represents homozygous α-thalassaemia, which is incompatible with life and results in a stillborn child with hydrops foet