A 10-year review of records of the King County Medical Examiner's Office found 87 deaths due to necrotizing fasciitis and related necrotizing soft tissue infections. In 64 of these cases there were sufficient details to provide an analysis of the manifestations, microbiology, and source of infection. One half (32) of the cases were due to injection of black tar heroin, the nearly exclusive form of heroin in the Northwest United States. Of those due to black tar injection, 24 were clostridial infections with various species represented, eight of which were Clostridium sordellii. Of the 32 cases not associated with drug injection, streptococcal species predominated, with Streptococcus pyogenes isolated in 14 cases. Only three of 32 cases not associated with injection drug use were clostridial infections. These differences were statistically significant. Staphylococcus aureus was isolated from 14 cases; two were methicillin-resistant strains. Overall, 28 of the 64 cases were polymicrobial infections, 15 due to black tar injection and 13 not associated with drug injection. This study supports the conclusion that necrotizing fasciitis due to black tar heroin injection is predominantly a clostridial disease, and in this way differs significantly from necrotizing fasciitis due to other causes.