Cooley's original description of β-thalassaemia major included marked bone deformities as a characteristic feature. These were thought to be due to expansion of haemopoiesis attempting to compensate for the congenital anaemia. Regular blood transfusions from infancy prevents these skeletal problems. Nevertheless, symptoms due to bone disease frequently occur in adult patients. Osteoporosis has not previously been reported as a cause of severe morbidity in thalassaemia major. The present study shows a high prevalence of low bone mass among thalassaemia major patients and analyses the predisposing causes. Bone density scans were performed in 82 patients with transfusion-dependent β thalassaemia. Factors known to be associated with low bone mass such as gender, endocrine disorders and lifestyle activities, together with factors specific to the thalassaemia and its management, were included in a series of univariate analyses to ascertain any significant associations. 42 (51%) of the patients had severely low bone mass and a further 37 (45%) had low bone mass. The three factors showing a statistically significant association with severely low bone mass were male sex, 24/38 (63%) males had severely low bone mass, compared with 18/44 (41%) females, the lack of spontaneous puberty, 22/32 (69%) who required therapeutic induction of pubertal development had severely low bone mass, compared with 19/47 (40%) with spontaneous puberty and diabetes, 8/10 (80%) diabetic patients had severely low bone mass, compared with 23/56 (41%) with normal glucose tolerance. There was no association between the bone mineral density measurements and the haematological characteristics or treatment details of these patients. Severely low and low bone mass are common findings in patients with β-thalassaemia major despite optimal transfusion and iron chelation. The associated features suggest that the severely low bone mass is due to endocrine abnormalities, in contrast to the haematological causes of bone disease characteristically seen in untreated thalassaemics.