It is well documented that the health of subjects with b-thalassaemia major or intermedia is complicated by an increased risk of diabetes mellitus and other endocrine disorders [1±7]. This is consequent to haemosiderosis, which is due to repeated blood transfusion and chronic haemolysis. However, little is known about the risk of diabetes mellitus, whether as a complication of treatment or of the underlying condition, in subjects with a-thalassaemia.Alpha-thalassaemia is absent or rare in Europe and Northern Asia and China [8,9]. In Southern China and Southeast Asia, the a-thalassaemia trait, haemoglobin H disease, and a-thalassaemia hydrops fetalis are found [8]. The incidence of the a-thalassaemia trait is about 6 % around Hong Kong [10,11], and up to 15 % in Southern China [9]. A recent study [12] confirmed that 5 % of the Chinese population in Hong Kong carry the a-thalassaemia trait. The overwhelming majority (4.5 %) are carriers of the Southeast Asian type of deletion, in which both a-globin genes on the same chromosome 16 are deleted (a-1 Diabetologia (2001) Abstract Aims/hypothesis. The purpose of this study was to examine the association between maternal a-thalassaemia trait and the occurrence of gestational diabetes mellitus in at-risk Chinese women in Hong Kong. Methods. From 3320 pregnant women who had delivered in our hospital and undergone the oral glucose tolerance test for various risk factors over a threeyear period, 163 with a-thalassaemia trait were identified (study group). The control group consisted of 163 women chosen from the next patient that was matched for maternal age and parity, following each index case. Comparison was made in the incidence of gestational diabetes mellitus defined by the World Health Organisation criteria, obstetric complications, and perinatal outcome. Results. The incidence of gestational diabetes mellitus was higher in the study group (62.0 % vs 14.7 %, p < 0.0001) which had a higher pre-pregnancy body mass index and lower haemoglobin concentrations. Although more patients in the study group had risk factors (41.7 % vs 26.4 %, p = 0.003), there was no difference in the pregnancy outcome or perinatal complications. Among the gestational diabetic women, those with a-thalassaemia trait were considerably younger and their infants had lower body mass index but there was no significant difference in the outcome. On multiple logistic regression analysis, the athalassaemia trait remained an important factor in the diagnosis of gestational diabetes (OR 11.74, 95 % CI 6.37±21.63). Conclusion/interpretation. Among women at risk of gestational diabetes, the presence of the a-thalassaemia trait is an additional risk factor for gestational diabetes mellitus. [Diabetologia (2001) 44: 966±971]