Sideropenic anemia is a common long-term complication of surgical bilio-pancreatic bypass for morbid obesity, and is frequently resistant to oral iron therapy. To study the pathogenesis of this phenomenon we investigated 7 such patients clinically and biologically, with special emphasis on iron absorption. Our results show that sideropenia, consistently present and frequently complicated by anemia, is due to deficient iron absorption and that this malabsorption is non-selective. Replacement therapy, when indicated, should therefore use the parenteral route.