To evaluate the long-term frequency and severity of anemia and selected vitamin and mineral deficiencies after gastric exclusion surgery for morbid obesity, the authors prospectively examined hematologic and nutritional parameters in 150 consecutive patients. These patients underwent a standardized gastric exclusion procedure during a six-year period (1976-1982) and were closely followed for up to seven years (mean, 33.2 months). Anemia developed in 36.8% of the population at a mean time from operation of 20 months. It was more frequent in women than in men (p less than 0.01), and it required transfusions in 3.5% of the population. A low serum iron concentration developed in 48.6%, iron deficiency in 47.2%, a low serum vitamin B12 concentration in 70.1%, vitamin B12 deficiency in 39.6%, and RBC folate deficiency in 18% of the population. Both iron and folate deficiencies responded to oral replacement. As a result of the high frequency and severity of anemia and nutritional deficiencies noted, all gastric exclusion patients should, as a minimum, be placed on oral multivitamin preparations containing iron, folate and vitamin B12. In addition, it is imperative that these patients be followed closely for the remainder of their lives with appropriate studies and replacement as necessary.