A molecular epidemiologic investigation was performed on a cluster of severe necrotizing Clostridium infections in 5 injection drug users admitted to an urban community hospital. Interviews with survivors suggested a point source of infection. Pulsed-field gel electrophoresis of SmaI restriction digests was performed to determine the molecular relatedness of clinically obtained isolates and isolates obtained from heroin samples and the home environment. A common clonal strain was found in Clostridium sordellii isolates from 2 socially unrelated patients and from drug paraphernalia. Clonality of a Clostridium perfringens strain from another patient isolate was identical to an isolate from a syringe found in her home. Other C perfringens isolates from patients, heroin, and the environment were determined to be polyclonal. We postulate that rapid recognition and public health notification led to rapid resolution of the outbreak.