The oral traditions of medicine and public health have it that malnutrition is an important risk factor for the development of tuberculosis (TB). Malnutrition profoundly affects cell-mediated immunity (CMI), and CMI is the principle host defense against TB. It makes biological sense. Although most health professionals readily accept this principle, much of this belief is based on uncontrolled observations such as disaster situations or on backwards logic from the cachexia common among TB patients. In fact, the evidence in humans is surprisingly thin from the perspective of scientific rigor. And few data, if any, quantify the extent of the relative or attributable risk of TB due to malnutrition. Moreover, until recently, data from experimental animals were based on animal models that were largely not relevant to human TB infection and disease. This article reviews the scientific data supporting the contention that malnutrition is an important risk factor for TB concentrating on observations in humans and on experimental animal studies based on a highly relevant animal model. If it is true, malnutrition may account for a greater population attributable risk of TB than HIV infection, and certainly a much more correctable one.