Regular blood transfusions from infancy until adulthood in β-thalassaemia major patients have substituted severe bone deformities with less marked skeletal lesions as osteoporosis. Osteoporosis is characterized by low bone mass and disruption of bone architecture, resulting in reduced bone strength and increased risk of fractures. Genetic factors have an important role in determining bone mineral density (BMD). We have investigated the possible association between BMD and two polymorphisms in 135 β-thalassaemic patients: (i) a substitution G→Τ in a regulatory region of the COLIA1 gene encoding for the major protein of bone (type 1 collagen), and (ii) a one-base deletion in intron 4 (713–8del C) of transforming growth factor beta 1 (TGF-β1) gene. We have found a remarkable incidence (90%) of osteopenia and osteoporosis among regularly transfused patients. Bone mass was lower in men than in women (P = 0·0023), with a more prevalent osteopenia/osteoporosis of the spine in men than in women (P = 0·001). The sample was stratified on the basis of BMD expressed as Z-score, i.e. normal, osteopenic and osteoporotic patients, and genotype frequencies of each group were evaluated. TGF-β1 polymorphism failed to demonstrate a statistical difference in BMD groups. However, subjects with heterozygous or homozygous polymorphism of the COLIA1 gene showed a lower BMD than subjects without the sequence variation (P = 0·012). The differences among genotypes were still present when the BMD was analysed as adjusted Z-score and when men and women were analysed separately (P = 0·022 and 0·004 respectively), with men more severely affected. Analysis of COLIA1 polymorphism could help to identify those thalassaemic patients at risk of osteoporosis and fractures.