Percentage decline over 10 year periods of bacillary tuberculosis related to mean age of 10 year cohorts of men (A) and women (B) in the period 1946 to 1974. The observations of tuberculosis disease in 10 year cohorts were collected from 1946, 1956 and 1966; and then from another set of cohorts in 1947, 1957 and 1967 etc. The percentage change within each 10 year period was recorded as percentage decline for each cohort from 1946 to 1956 and from 1956 to 1966 etc. The figure derived from the observation of the cohort of 20-29 years in 1946 being 30-39 years in 1956 is represented as a single point in the figure under the mean age of 30 in the observation period. The figures of this age group was derived using independent observation of tuberculosis disease in the years of 1946-56, 1947-57, 1948-58 etc Each point therefore represents the observed decline within each 10 year period for every birth cohort as calculated from the rates given by the National Tuberculosis Registry and Statistics Norway. Linear curve estimation was made using the regression module of SPSS version 15.0 with the mean age of 10 year cohorts in 10 year periods as the independent variable. The curve estimations were significant with P < 0,0005 and the estimated declines were -0,882 for men (A) and -1,044 for women (B).