The purpose of this study was to examine school age children's knowledge of bicycling rules of the road and their bicycling behaviors. A one-page questionnaire was administered in the classroom to 276 of 300 children in grades 4 through 8 of an upper middle class suburban school district. The children were questioned regarding their knowledge of three basic bicycling rules of the road, prior bicycle safety instruction, use of bicycle helmets, and the occurrence and severity of previous bicycle accidents. Students who reported receiving previous bicycle safety instruction were more knowledgeable than those receiving no instruction regarding rule 2, always stop at a stop sign or red light (90% compared with 74%), and rule 3, always stop and look when approaching a street from a driveway or alley (74% compared with 60%). Students who did not know rule 3 were more likely to have had a recent bicycle accident in which their bicycle was damaged (21% compared with 8%) and to have ever gone to the hospital or a physician because of injuries sustained in a bicycle accident (19% compared with 9%). Children who lacked knowledge of basic bicycling rules were more likely to have had a significant bicycling accident. Bicycle safety instruction increases children's knowledge of these rules and should be promoted by physicians caring for children.