Young drivers have a disproportionately high risk of experiencing a road traffic crash. On 1 August 1987 a Graduated Driver's Licensing System (GDLS) was introduced in New Zealand. This system was designed to give young drivers (i.e. 15-24 years inclusive) experience in driving while being excluded from high risk driving situations. We sought to determine the impact of the GDLS on serious injury crashes. The source of the injury crash data was New Zealand's Health Information Services' national public hospital inpatient morbidity data files for the years 1979-1992 inclusive. We disaggregated the occupant data into three age groups 15-19 years, 20-24 years, and persons 25 years of age or older and compared their trends in injury. In order to determine whether the incidence of motor vehicle crashes was simply following trends in other injury events we also included two 15-19 year old non-traffic injury comparison groups. Using time series analyses we showed that the introduction of the GDLS was closely followed by substantial reductions in car crash injuries for all age groups, especially 15-19 year olds (23% reduction). After considering effects for older occupants we speculate that the effect is likely to be substantially less than 23%. An analysis of licensure data suggests that the reduction in crashes may, in large part, be attributable to an overall reduction in exposure.