Mandatory motorcycle helmet-use legislation is supported by the high morbidity of motorcycle trauma and its cost to society. Opponents argue, however, that the majority of motorcycle trauma morbidity and costs are the result of injuries to body regions other than the head. Previous data do not address this argument because they fail to control for differences in non-head injury severity (i.e., kinetic impact) between helmeted and unhelmeted patients. This study investigates the impact of helmet use on the morbidity and cost of motorcycle trauma, after controlling for non-head injuries. A retrospective review of all patients admitted to Harborview Medical Center with motorcycle trauma from 1/1/85 to 1/1/90 was performed. Non-head injury severity was determined by calculating an ISS that did not include head injury. This non-head ISS was used to control for injury severity below the neck. Four hundred twenty-five patients were identified. Stratified analysis showed that helmet use decreased the need for and duration of mechanical ventilation, the length of ICU stay, the need for rehabilitation, and prevented head injury. Costs of acute care were significantly less in helmeted patients. Regression analysis, controlling for age, gender, and blood alcohol level (as well as non-head injury severity), confirmed that acute costs were 40% less with helmet use.