Among various epidemiological indices relative to tuberculosis, the most valuable index is the annual risk of infection: this expresses the risk in the population at the present time, while the prevalence rate is the result of infection in the past. Estimates of this risk were calculated (by means of a simple mathematical model) on the basis of various French epidemiological data: prevalence of infection was calculated when tuberculin surveys were carried out in groups of children from the Paris area (Child Care Centre in Paris in 1934, 1943, 1953 and 1961), and in students (all Universities in 1947). The results show an important diminution of this risk which, calculated from the given data, has the following values: 880 and 550 per 100,000 in 1960, 310 and 233 in 1970. In assuming that its further evolution will conform to the present tendency, it is possible to evaluate the risk for 1980 as 112 and 100 per 100,000 respectively.