An illness characterized by a severe infectious syndrome has affected at least 88 drug users, 40 of whom have died, in Scotland, England, and Ireland since April 1, 2000. The affected drug users had injected heroin subcutaneously or intramuscularly. Case-control studies are underway in Scotland and Ireland to identify the risk factors associated with this outbreak. Currently, no specific type or source of heroin has been identified as the cause. (R.A.)