- 200 patients after Roux-en-Y gastric bypass were nutritionally followed for an average of 6.7 years. They were randomly selected out of 450 patients operated on during 1980-1985. They had lost a mean of 56 +/- 22% of their excess weight and had remained stable in weight for the preceding 5 years with limited nutritional caloric restriction. Iron saturation, hemoglobin and mean corpuscular volume declined gradually and significantly (p less than 0.0001, p less than 0.0001 and p less than 0.001, respectively). However, mean serum concentration of vitamin B12, which also had decreased significantly during the preceding 5 years (p less than 0.0004), increased during recent years, but still was lower than before the bypass. Albumin and transferrin levels remained normal. There was no correlation between weight change and serum concentration of iron or vitamin B12. We conclude that 6.7 years after surgery, while there is no caloric or protein deficiency, there has been gradual development of iron deficiency, but not correlated with weight loss.