A study of the injuries sustained by 1,892 bicycle riders during accidents indicated that 432 of the bicyclists had been wearing a helmet and 64 of the latter group had sustained an impact to the helmet. The 64 helmets were evaluated in this project to relate the nature and severity of the impact they had sustained to the head injury experienced by the wearer. The protective performance of the helmet shells, impact absorbing liners, and retention systems were evaluated, and the severity of the impacts sustained by the helmets was simulated in the test laboratory. The simulation was performed by dropping sample helmets from progressively greater heights in a test apparatus until the damage observed on a sample helmet matched that observed on an accident damaged helmet. The severity observed in the simulated impacts was compared with the severity of test impacts prescribed in established helmet performance standards (ANSI 1984; Snell 1984; AS 1986). It was found that all of the impacts occurred against flat objects; a high proportion of helmets sustained more than one impact; most impacts occurred on areas of a helmet which were not tested during certification to a standard; and many impacts were more severe than those stipulated in performance standards. The predominant form of head injury recorded was low severity concussion--AIS-1, AIS-2, AIS-3. All serious head injuries occurred when the helmet came off the rider's head and collapsed due to a material defect or was struck predominantly below the rim. A high proportion of helmets worn by young riders had been misused, and many helmets displayed defects in the impact-absorbing liners. Recommendations have been made for improving helmet construction and altering current standards to reflect the conditions encountered in the field.