Patients with thalassemia major often report that they do not maintain their usual pretransfusion hemoglobin concentration during the summer season. We collected 3977 pretransfusion hemoglobin values, amount of blood transfused, and intertransfusion intervals for 94 patients with thalassemia major from 4 centers worldwide. We also assessed the hematocrits of blood donors, the hemoglobin content of units transfused in one center, and the local mean monthly temperatures during the periods of data collection. Pretransfusion hemoglobin levels were significantly lower during the summer in all centers except the one center where monthly temperatures have the least variation throughout the year. A similar relationship to temperature was observed for the hematocrits of blood donors and the hemoglobin content of donor units. This study confirms that pretransfusion hemoglobin levels in patients with thalassemia major are lower in the summertime. Possible mechanisms include expansion of plasma volume with resultant hemodilution in the patient and lower hemoglobin content in donor blood.