The results of studies performed by the General Motors Research Laboratories to investigate the relationship between car size (indicated by car mass) and safety are presented. Most of the studies reviewed rely heavily on one data set that has uniquely useful features. This is the Fatal Accident Reporting System (FARS) data set maintained by the National Highway Traffic Safety Administration (NHTSA), which is part of the U.S. Department of Transportation. This file contains detailed information on every fatal traffic crash occurring in the United States since 1975. The studies find that given a single-car crash, the unbelted driver of a 900kg car is about 2.6 times as likely to be killed as is the unbelted driver of an 1,800 kg car. The relative disadvantage of the smaller car is essentially the same when the corresponding comparison is made for belted drivers. For two-car crashes, it is found that the driver of a 900kg car crashing into another 900kg car is about 2.0 times as likely to be seriously or fatally injured as is the driver of an 1,800kg car crashing into another 1,800kg car. The above-mentioned results refer to what happens if a crash occurs. Relations between car size and accident involvement rates will also be discussed.