The authors studied the relationship between mortality and illegal drug use. The study was conducted over a 4-year period between 1995 and 1999. Research subjects were clients referred to the National Treatment Outcome Research Study (NTORS). Data were collected from intake interviews with drug treatment program clients and the review of death certificates and post-mortem examinations of the members of the study population who died. The data indicated that mortality rates within the study population were 1.2 percent higher per year than those for age-matched non-drug users. The majority, 68 percent, of deaths were due to drug overdoses, 18 percent were attributable to other illness, and 14 percent were attributable to self-injury, accident, or violence. Polydrug use was identified as an increased mortality risk factor with alcohol and benzodiazepines or methamphetamine combinations being particularly lethal. Other risk factors included anxiety and homelessness. The authors recommend that drug treatment programs invest more resources educating clients about the dangers of polydrug use. To increase the understanding of the risks of polydrug use the authors also advocate the listing of all substances found during post mortem examinations be listed on the final death certificate. 3 tables, 50 references