In June 1998, six unexpected anterior cruciate ligament ruptures within 12 months were detected by routine injury surveillance in a cohort of Australian Army recruits. A systematic investigative process was used to identify the probable cause. Review of the case histories, followed by on-site assessment of the common hazard, revealed that all ruptures occurred as recruits twisted or landed on newly laid rubber matting at one obstacle course. The matting was laid with the intention of countering ground wear, shock of impact, and slipping to reduce the risks of ankle sprain and stress fracture. Review of the literature indicated that the interface between the rubber boot sole and rubber matting was associated with a very high coefficient of friction, probably causing excessive torque at the knee. Similar mechanisms of anterior cruciate ligament injury have been reported in sports and skiing. A subsequent study of intervention effects, reported separately in this issue, was conducted to confirm the damaging role of the matting. Routine injury surveillance and systematic investigation are valuable tools for injury prevention. Clinicians are well equipped for both.