The use of seat belts by drivers was measured in three target and three control cities in Illinois before, during, and after the application of heightened police enforcement of the seat belt law in the target cities. The extra enforcement produced increases in the use of seat belts by drivers during the four months of the heightened enforcement. Observations of seat belt use continued for four months after the end of the enforcement, and a substantial residual effect remained in two of the three target cities. In one target city the use of seat belts had dropped close to the level at the start of the project by the end of the second month after the end of enforcement. Taking account of the percentage of drivers who became aware of the increased enforcement suggested that a sustained program of enforcement may result in 60%-70% of drivers using seat belts.