A series of morbidly obese patients was treated surgically with a gastric bypass, and a subsequent series received a vertical banded gastroplasty. To compare some of the nutritional effects of these two procedures we measured serum vitamin B12 levels, absorption of food vitamin B12, frequency of microcytosis of erythrocytes, and frequency of anemia at one or more years after surgery. Patients with a gastric bypass showed greater weight loss, a greater frequency of microcytosis and anemia, more frequent subnormal serum levels of vitamin B12, and impressive failure to absorb food vitamin B12. Boiling the food containing vitamin B12 led to increased absorption.