Adaptation of vitamin B12 absorption by small intestine has been suggested by experimental and clinical studies. Five partial ileal bypass patients and ten jejunoileal bypass patients were studied for adaptation of B12 absorption following surgery. Adaptation of vitamin B12 absorption could not be demonstrated in either group. Few of the patients on an individual basis demonstrated adaptation. Parenteral B12 should be given to all patients who undergo bypass surgery unless evidence of persistent normal B12 absorption has been obtained.