The rare association of alpha thalassaemia and mental retardation has been described previously. Molecular studies of the alpha globin cluster in these cases have been heterogeneous, with some patients having large deletions while in others the alpha globin complex appears to be intact (non-deletional). The non-deletional cases form a distinct group whose features include severe mental retardation, haematological changes of haemoglobin H (Hb H) disease, developmental defects, and unusual patterns of inheritance. To date, five cases have been described with non-deletional alpha thalassaemia-mental retardation. We present here a further example of a young male of Northern European origin who appears to have the non-deletional form of the disease. Clinical features included severe mental retardation, Hb H disease, and developmental defects similar to those reported previously. DNA mapping, including pulsed field electrophoresis, showed no evidence of deletions within the alpha globin cluster. Karyotypic analysis indicated an increase in random breakage, which has been observed previously in one case of deletional alpha thalassaemia-mental retardation. Profuse Hb H bodies and Hb H on electrophoresis were consistent with Hb H disease. However, the latter was present at a relatively low level (1.6%) and, as well, the mean corpuscular volume (82.8 fl) and mean corpuscular haemoglobin (26.4 pg) were surprisingly high. Our findings are compared to other cases described with the non-deletional Hb H-mental retardation syndrome.