To identify bicycle-riding circumstances associated with bicycle-related injury among school-aged children. Case-control. One metropolitan emergency department and 3 suburban emergency departments. Consecutive sample of children aged 7 through 18 years who experienced bicycle-related trauma and control children seen for non-bicycle-related trauma (matched for age within 1 year, sex, and area of residence [urban vs suburban]). Parents and case children were interviewed by telephone about the bicycle ride resulting in their visit to the emergency department. Parents and control children were interviewed about their most recent bicycle ride. The survey instrument addressed the following potential risk factors: helmet use, bicycle speed, road conditions, riding location, bicycle condition, an adult presence, riding destination, bicycle style, and stunt riding. Interviews were completed with 47 (73%) of 64 eligible case children and 42 (69%) of 61 control children with the following age distribution: 27 (30%) of the interviews were completed with children aged 7 to 9 years, 40 (45%) of the interviews were completed with children aged 10 to 14 years, and 22 (25%) of the interviews were completed with children aged 15 to 18 years. Fourteen children (16%) were wearing helmets. There was a high degree of agreement between parent and child responses, higher for case children than for control children. In univariate analyses, injury was associated with riding with other children (vs riding alone or with adults), riding fast or slow (vs normal speed), riding a BMX-style (motocross) bicycle (vs another standard or multispeed style bicycle), playing on the bicycle (vs going to school or other purposeful or nonpurposeful trip), and riding only on the sidewalk (vs in the street). More case children than control children were farther than 3/4 mile (> 1.2 km) from home (38% vs 19%, P = .05). Multiple logistic regression identified' slow riding speed (odds ratio, 10.3;95% confidence interval, 1.6-66.8), distance from home farther than 3/4 mile (> 1.2 km) (odds ratio, 3.7;95% confidence interval, 1.1-12.5), and riding on the sidewalk (odds ratio, 6.1;95% confidence interval, 1.8-20.5) as independent risk factors for injury. This study identifies 3 counterintuitive but apparently strong behavioral risk factors for bicycle injuries treated in an emergency department in children aged 7 through 18 years in the Chicago (III) area. These findings will need to be confirmed in larger samples from a wider range of locales. In addition to stressing the importance of wearing a helmet when riding a bicycle, it may be desirable to include the findings of this study in anticipatory guidance discussions with school-aged children.