We herein report two cases of clinically diagnosed X-linked sideroblastic anemia with subsequent diabetes mellitus. Cases 1 and 2 are brothers, 69 and 61 years old, who both developed anemia and secondary hemochromatosis. Case 1 was first diagnosed with diabetes mellitus at the age of 63, and insulin therapy was initiated 5 years later. At this point, his urinary C-peptide levels were 257 μg/day. Approximately one year later, he was admitted to the hospital to have his diabetes management regimen adjusted. The urinary C-peptide levels were found to have fallen to 15.2 μg/day, and abdominal CT showed atrophy of the pancreas. Case 2, the younger brother, had more severe anemia and had also required frequent transfusions since adolescence. He developed liver cirrhosis, hepatocellular carcinoma and left ventricular diastolic dysfunction. His diabetes was first diagnosed at the age of 54, and insulin therapy was initiated. At the age of 61, his insulin deficiency had progressed. X-linked sideroblastic anemia is a rare hereditary form of anemia, and diabetes mellitus as its complication is not well-documented. In our two cases, both developed insulin deficiency, but Case 1 showed pronounced insulin resistance initially before he developed pancreatic β-cell dysfunction.