The question of whether iron overload causes glucose intolerance was studied in a group of 26 multiply transfused homozygous beta thalassemics. Of the 26, 13 (50%) had some abnormality in their oral glucose tolerance test, 5 fitting criteria for definite diabetes. Glucose intolerance correlated significantly with number of transfusions received and with age of the subjects, while a positive family history for diabetes was more common in the subjects with glucose intolerance. These data and a chart review of four deceased thalassemics with overt diabetes are consistent with the following conclusions: (1) glucose intolerance is common in multiply transfused thalassemics; (2) the incidence of abnormal glucose tolerance correlates with the number of transfusions received and the age of the subject; (3) a family history of diabetes may predisose multiply transfused thalassemics to glucose intolerance.