Arson is a violent crime and a public health problem that causes injuries and deaths, destroys homes, and destabilizes neighborhoods. During the late 1970s, pre-Halloween pranks in Detroit, Michigan, turned destructive when hundreds of fires were set deliberately throughout the city; in 1984, a record of 810 fires were set during the Halloween period. In 1985, a city wide anti-arson campaign that involved the mobilization and training of thousands of community volunteers was begun in Detroit. This report describes the multiple components of the anti-arson intervention from 1985 through 1996 and changes in the incidence of Halloween fires. Both the decrease in annual Halloween arson fires after the intervention began and the inverse relationship between the number of volunteers and the number of fires suggest a causal effect. This study illustrates the capacity of an urban community to mobilize its residents and stakeholders, the importance of community participation and multisectoral partnerships in program planning and implementation, and the challenges faced in retrospectively evaluating an apparently successful, complex, community-based intervention.