In the past, evaluation of helmet efficacy has been based on laboratory tests of limited relevance to real crashes. In the present study 894 South Australian bicycling enthusiasts returned mail questionnaires about their most recent bicycle crash and their helmet use at the time. 197 bicyclists reported a crash within the past five years in which they had struck their head or helmet. Helmet status at the time of the crash was reported as: no helmet used (n = 75), hairnet-style helmet (n = 69), hard-shell with soft or no liner (n = 37), or hard-shell helmet with stiff liner (n = 16). Analysis of the crude, unadjusted data showed a statistically significant association between helmet use and reduced severity of head injury. The association persisted after adjustment for age and sex of rider, and severity of crash forces. Using an unpublished method developed by Somers, it was estimated that the risk of death from head injury was considerably reduced for helmeted relative to unhelmeted bicyclists, depending on helmet type.