A case-control study was conducted in El Paso County, Colorado to estimate differences in risk of head injury among persons in motorcycle crashes who were or were not wearing helmets. There were 71 cases, motorcyclists with head injuries from crashes, and 417 controls, motorcyclists in crashes without head injuries. Motorcyclists not wearing helmets were 2.4 times as likely to sustain head injuries (95% confidence limits: 1.23, 4.70) than motorcyclists wearing helmets. This odds ratio was adjusted for age and crash characteristics, using logistic regression. Alcohol intoxication and severity of motorcycle damage were also associated with significantly elevated odds ratios related to sustaining a head injury.