Though injury incidence and hospitalisations are likely to be correlated, a range of factors other than incidence of injury in a population may influence trends in hospitalised injuries. These include technical changes in the hospital data's coverage, and real changes in the incidence of hospitalisations independent of the population incidence. This paper addresses the latter using the example of traumatic brain injury (TBI) hospitalisations in New Zealand. Data were New Zealand public hospitals inpatient discharges. Five measures of TBI severity were used. The relative rate of minor to serious TBI hospitalisations declined by approximately 2-6% per year from 1988 to 1998. This decline is observed across different mechanisms and the two severity measures used in the detailed analysis. The relative decline in minor to serious TBI is likely to be related to a change in the probability of admission rather than a change in the population TBI incidence. As most TBI hospitalisations are minor this suggests the trend in TBI hospitalisations was significantly influenced by factors other than changes in population incidence. Any analysis of routinely collected secondary injury data needs to consider case selection carefully, especially if trends are being examined. Applying a severity threshold should give more reliable trends.