Results of an accident investigation project conducted by the Indiana University Institute for Research in Public Safety for the National Highway Traffic Safety Administration are presented. The study focused on the statistical determination of the relative roles played by human, environmental and vehicle deficiencies in automobile accident causation. This paper emphasizes the examination of the role of the vehicle.The study was based in Monroe County, Indiana. Data were collected on three levels: baseline data were assembled to allow definition of the project universe; accidents were investigated on-site at the time of occurrence by technicians; and a sample of these accidents was independently examined by a multidisciplinary team. Causative factors were identified and ranked.Vehicle-related deficiencies were identified as having a probable involvement as either causative or severity-increasing factors, in not less than 14% of accidents investigated. A definite involvement as causative factors was established in not less than 6% of accidents investigated.A comparison of component outage rates of the accident-involved vehicles and vehicles of the nonaccident population is also presented.