Clinically, Hb H disease presents as alpha thalassaemia intermedia. The majority have a clinical course of well compensated chronic haemolytic anaemia with exacerbations caused by oxidant haemopoietic stress. Haematological and DNA studies have identified the common molecular defects associated with Hb H disease in West Malaysia. Patients were shown to have α0 - thalassaemia of the South-East Asian type (--/SEA) in association with α+ thalassaemia or with Hb Constant Spring. In a study of 32 patients with Hb H disease, 50% were shown to have α+ thalassaemia (-α 3.7, 87.5%; -α 4.2, 12.5%). Growth and development were normal in all patients, and cholelithiasis was seen in 50% of the patients with the non-deletional type of Hb H disease. Comparisons of the clinical parameters (the necessity of blood transfusion, thalassaemia facies), haemoglobin and bilirubin levels show that the non-deletional type of Hb H disease in West Malaysia is associated with a more severe clinical state than the deletional type.