Evaluation of injury prevention programs is critical for measuring program effects on reducing injury-related morbidity and mortality or on increasing the adoption of safety practices. During the planning and implementation of injury prevention programs, evaluation data also can be used to test program strategies and to measure the program's penetration among the target population. The availability of this early data enables program managers to refine a program, increasing the likelihood of successful outcomes. The Oklahoma City Smoke Alarm Project illustrates how an evaluation was designed to inform program decisions by providing methodologically sound data on program processes and outcomes. This community intervention trial was instituted to reduce residential fire-related injuries and deaths in a geographic area of Oklahoma City that was disproportionately affected by this problem. The distribution of free smoke alarms in targeted neighborhoods was accompanied by written educational pamphlets and home-based follow-up to test whether the alarms were functioning correctly. Early evaluation during the planning and implementation phases of the program allowed for midcourse corrections that increased the program's impact on desired outcomes. During the six years following the project, the residential fire-related injury rate decreased 81% in the target population but only 7% in the rest of Oklahoma City. This dramatic decline in fire-related injuries in the target area is largely attributed to the free smoke alarm distribution as well as to educational efforts promoting awareness about residential fires and their prevention.