Prevention tools are challenged by risky behaviors that follow their adoption. Speed increase following helmet use adoption was analyzed among bicyclists enrolled in a controlled intervention trial. Speed and helmet use were assessed by video (2621 recordings, 587 participants). Speeds were similar among helmeted and nonhelmeted female cyclists (16.5 km/h and 16.1 km/h, respectively) but not among male cyclists (helmeted: 19.2 km/h, nonhelmeted: 16.8 km/h). Risk compensation, observed only among male cyclists, was moderate, thus unlikely to offset helmet preventive efficacy.