In this study, 888 injury-producing bicycle-motor vehicle collisions were examined. On the basis of the movements of the vehicles involved, the bicyclist or the bicycle or both was probably responsible for the initiation of more than three fourths of the collisions. Bicyclist age was strongly related to probable responsibility for the collision. Through age 12, nine out of ten bicyclists were probably responsible for the collision; above age 12, probable responsibility decreased in proportion to age; and only 34% of the bicyclists aged 25 years or older were probably responsible. Collisions involving bicyclists in various age groups differed considerably in where, when, and how they occurred. The movements of the vehicles primarily involved in the initiation of the collisions were evaluated in terms of counter-measure planning.