OBJECTIVES: This study evaluated factors associated with accidental fatal drug overdose among a cohort of injection drug users (IDUs). METHODS: In a prospective cohort study of 2849 IDUs in King County, Washington, deaths were identified by electronically merging subject identifiers with death certificate records. Univariate and multivariate Cox regression analyses were performed to identify predictors of overdose mortality. RESULTS: Thirty-two overdoses were observed. Independent predictors of overdose mortality were bisexual sexual orientation (relative risk [RR] = 4.86; 95% confidence interval [CI] = 2.30, 13.2), homelessness (RR = 2.30; 95% CI = 1.06, 5.01), infrequent injection of speedballs (RR = 5.36; 95% CI = 1.58, 18.1), daily use of powdered cocaine (RR = 4.84; 95% CI = 1.13, 20.8), and daily use of poppers (RR = 22.0; 95% CI = 1.74, 278). CONCLUSIONS: Sexual orientation, homelessness, and drug use identify IDUs who may benefit from targeted interventions.