In order to describe and analyse the development of the incidence of traumatic brain injury (TBI) in Denmark for different age groups of the two genders from 1979 through 1993 (for fatal injuries through 1996), a computerised search corresponding to diagnoses ICD 8th ed., 800, 801, 803, 850–854 from 1979 through 1993 was carried through in the national hospital register. Each person was counted only once, according to the most serious injury during the study period. For fatal cases, the search was extended till 1996. From 1979–1981 to 1991–1993, the total age-adjusted incidence of persons hospitalised under diagnoses ICD 800, 801, 803, 850–854 decreased 41% from 265 to 157 per 100,000 of the population per year. Decreases were 42% for ICD 850, brain concussion, 56% for ICD 800, 801, 803, cranial fractures, and 16% for ICD 851–854, structural brain injury. The percentage of cases with ICD 851–854 increased from 8.4 to 11.7% of the total. From 1979–1981 to 1985–1987 there was a 2% decrease in fatal TBI in and outside hospital (from 14.68 to 14.35 per 100,000), against a total 27% decrease to 10.67 in 1994–1996. For diagnoses ICD 851–854 and for fatal cases, significantly accelerating decreases from 1985–1987 were found only for the younger age groups. Consequently, in the period from 1979 to 1993, the mean age at injury increased by 10 years for persons hospitalised under diagnoses ICD 851–854. Decreases may be explained partly by changing admission and other hospital practices, and partly by the effect of comprehensive national preventive programs launched at the middle of the study period, the effect of which seemed to vary by age group and gender.