This paper examines the effectiveness of seat belts in reducing injury among passenger car drivers and right front passengers. The analysis is based on more than 1.5 million occupants involved in North Carolina crashes during the years 1973-1981. Initial results show that seat belts reduce the risk of serious injury to the driver by 58% and fatal injury by 73%. For right front passengers, the comparable figures are 53% and 66%. When a measure of vehicle deformity reflecting the severity of the crash is introduced as a control variable, these effectiveness values decline somewhat. Depending on the specific approach taken, seat belts are shown to reduce the risk of serious injury to passenger car drivers by 51-52%, and the risk of fatal injury by 63-67%. For right front passengers, the effectiveness ranges are 43-44% for serious injury and 53-55% for fatal injury. While these adjusted estimates of belt effectiveness are lower than those based on the raw data, they nevertheless represent considerable benefit to car occupants using seat belts.