Aggressivity of a vehicle is defined as the fatality or injury risk for occupants of other vehicles with which it collides. Because of the strong effect vehicle weight has on this risk, gross aggressivity which includes the effect of weight, and net aggressivity which excludes the effect of weight are distinguished. Data from the Fatal Analysis Reporting System (FARS) and the General Estimates System (GES) for 1991 to 1994 were used for fatalities, and for crash involvements, respectively. The relation between weight and wheelbase of cars was studied, and the concept of "overweight" introduced. For collisions between two cars, the relation of fatality risks with car weight, overweight, wheelbase and bumper height were studied. Also, adjustments were made for the higher vulnerability of older victims. Collisions between cars and light trucks - including utility vehicles, pickup trucks and vans - were studied. A limited analysis of the effect of vehicle weight was performed. The driver fatality risks in collisions between cars and light trucks were studied by collision configuration. In all cases, the risk for car drivers was much higher than for drivers of light trucks.