The association between body mass index (BMI) and incidence of tuberculosis was studied among 1,717,655 persons older than 14 years who were followed for 8-19 years. The incidence of pulmonary tuberculosis decreased with increasing BMI. The trend was roughly linear on the log scale, and was the same for both sexes and all age groups. X-ray-positive (calcification, pleural changes, infiltration) individuals showed the same trend, but on a much higher level than X-ray-negative individuals. When the total observation period was subdivided according to duration, the trend was found to be only slightly less steep for 10-19 years than for 0-4 and 5-9 years. When the material was divided simultaneously by group of observation years and by miniature X-ray findings, the association persisted. No association was seen between the incidence of other forms of tuberculosis and BMI. For persons reported with tuberculosis, a slightly decreasing mortality of tuberculosis was seen with increasing BMI, while mortality, all causes, seemed to have a slightly U-shaped association with BMI.