Objectives:​ To examine the protective effectiveness of bicycle helmets in 4 different age groups of bicyclists, in crashes involving motor vehicles, and by helmet type and certification standards.

Research Design:​ Prospective case-control study

Setting:​ Emergency departments (EDs) in 7 Seattle, Wash, area hospitals between March 1, 1992, and August 31, 1994.

Participants:​ Case subjects were all bicyclists treated in EDs for head injuries, all who were hospitalized, and all who died at the scene. Control subjects were bicyclists treated for nonhead injuries.

Main Results:​ There were 3390 injured bicyclists in the study;​ 29% of cases and 56% of controls were helmeted. Risk of head injury in helmeted vs unhelmeted cyclists adjusted for age and motor vehicle involvement indicate a protective effect of 69% to 74% for helmets for 3 different categories of head injury:​ any head injury (odds ratio [OR], 0.31;​ 95% confidence interval [CI], 0.26-0.37), brain injury (OR, 0.35;​ 95% CI, 0.25-0.48), or severe brain injury (OR, 0.26;​ 95% CI, 0.14-0,48). Adjusted ORs for each of 4 age groups (<6 y, 6-12 y, 13-19 y, and ≥20 years) indicate similar levels of helmet protection by age (OR range, 0.27-0.40). Helmets were equally effective in crashes involving motor vehicles (OR, 0. 95% CI, 0.20-0.48) and those not involving motor vehicles (OR, 0.32;​ 95% CI, 0.20-0.39). There was no effect modification by age or motor vehicle involvement (P=.7 and P=.3). No significant differences were found for the protective effect of hard-shell, thin-shell, or no-shell helmets (P=.5).

Conclusions:​ Bicycle helmets, regardless of type, provide substantial protection against head injuries for cyclists of all ages involved in crashes, including crashes involving motor vehicles.