Mortality from pulmonary tuberculosis in the United States was analyzed by cohort. The introduction of effective chemotherapy necessarily renders models based on generation differences alone inappropriate in this disease. However, such models continue to be used. The data show major departures from the prediction of a generation based model during the 1950's. Projection of the 1941 rates and cohort slopes to 1970 using a generation model predicts 358,000 more deaths than were actually certified. The departure was smaller for blacks than for whites, and differences in delivery of treatment probably account for this. Projections of mortality need to be made for the planning of control measures. However, such projections must be done with the understanding that inter-cohort differences can be altered during adult life.