Objective: To determine the effect of helmets on the risk of head and neck inj uries in skiers and snowboarders.Design: Matched case-control and case crossove r study.Setting: 19 ski areas in Quebec, Canada, November 2001 to April 2002. Pa rticipants: 1082 skiers and snowboarders (cases) with head and neck injuries rep orted by the ski patrol and 3295 skiers and snowboarders (controls) with non-he ad or non-neck injuries matched to cases at each hill. Main outcome measures: E stimates of matched odds ratios for the effect of helmet use on the risk of any head or neck injury and for people requiring evacuation by ambulance. Results: T he adjusted odds ratio for helmet use in participants with any head injury was 0 .71 (95%confidence interval 0.55 to 0.92), indicating a 29%reduction in the ri sk of head injury. For participants who required evacuation by ambulance for hea d injuries,the adjusted odds ratio for helmet use was 0.44 (0.24 to 0.81). Simil ar results occurred with the case crossover design (odds ratio 0.43, 0.09 to 1.8 3). The adjusted odds ratio for helmet use for participants with any neck injury was 0.62 (0.33 to 1.19) and for participants who required evacuation by ambulan ce for neck injuries it was 1.29 (0.41 to 4.04). Conclusions: Helmets protect sk iers and snowboarders against head injuries. We cannot rule out the possibility of an increased risk of neck injury with helmet use, but the estimates on which this assumption is based are imprecise. Objective: To determine the effect of helmets on the risk of head and neck inj uries in skiers and snowboarders. Design: Matched case-control and case crossover r study. Letting: 19 ski areas in Quebec, Canada, November 2001 to April 2002. Pa rticipants: 1082 skiers and snowboarders (cases) with head and neck injuries rep orted by the ski patrol and 3295 skiers and snowboarders (controls) with non-he ad or non-neck injuries matched to cases at each hill. Main outcome measures: E stimates of matched odds ratios for the effect of helmet use on the risk of any head or neck injury and for people requiring evacuation by ambulance. Results: T he adjusted odds ratio for helmet use in participants with any head injury was 0.71 ( 95% confidence interval 0.55 to 0.92), indicating a 29% reduction in the ri sk of head injury. For participants who required evacuation by ambulance for hea d injuries, the adjusted odds ratio for helmet use was 0.44 (0.24 to 0.81). Simil ar results occurred with the case cr The adjusted odds ratio for helmet use for participants with any neck injury was 0.62 (0.33 to 1.19) and for participants who required evacuation by ambulan ce for neck injuries it was 1.29 (0.41 to can 4.04). Conclusions: Helmets protect skiers and snowboarders against head injuries. We can not rule out the possibility of an increased risk of neck injury with helmet use, but the estimates on which this assumption is based on imprecise.