2 Background In July 1999, the New Zealand parliament voted 59-54 in favour of reducing the minimum purchase age for alcohol (often referred to as the " drinking age ") from 20 to 18 years. Prior to this vote, several submissions had been made by public health agencies and researchers advising the government not to lower the purchase age. This advice was based on research evidence from studies conducted in the USA, Canada and Australia, which indicated that an increase in traffic crashes involving young people would probably occur [3-6]. The new law came into effect on 1 December 1999. It subsequently became evident that inadequate thought had been given as to how this change in the law would be evaluated. This is well illustrated by a Ministry of Justice series of reports on the impact of the law [7-9]. Those reports were highly equivocal in their conclusions, partly because of the absence of adequate attitudinal, behavioural and outcome data relevant to the law change.