To determine whether the high incidence of tuberculosis among Asiatic Indians in the United Kingdom was due to impaired killing of Mycobacterium tuberculosis by macrophages from patients deficient in cobalamin, a nutritional survey was carried out among 1187 Indians and the incidence of tuberculosis determined from medical records. The question asked was whether tuberculosis was significantly more common among life-long vegetarians compared with omnivores. The incidence of tuberculosis in vegetarians was 133 in 1000 and that in subjects on mixed diets 48 in 1000. These findings lend support to the hypothesis that dietary factors are of major importance in determining the susceptibility of Asiatic Indians to tuberculosis.