Twelve months before the wearing of a cycle helmet was to become mandatory in New Zealand, a substantial proportion of cyclists on public roads had 'voluntarily' adopted wearing a helmet. Helmet wearing rates had increased up to 84, 62 and 39% for primary school children, secondary school children, and adults respectively by the end of the period of interest. The purpose of this study was to examine the serious injury trends for three age groups of cyclists: primary school age (5-12 years), secondary school age (13-18 years), and adults (over 18 years) admitted to selected public hospitals between 1980 and 1992; twelve months before the introduction of helmet legislation. Serious injury was defined as 'admitted to hospital' then disaggregated by type of crash and length of stay. Statistical models were constructed that included the proportion of people admitted to hospital with a head injury, then analysed using Poisson regression. Results revealed that the increased helmet wearing percentages has had little association with serious head injuries to cyclists as a percentage of all serious injuries to cyclists for all three groups, with no apparent difference between bicycle only and all cycle crashes. Discussion of the results includes possible explanations for the absence of a decline in the percentage of serious head injury among cyclists as cycle helmet wearing has increased.