During 1999-2006, the number of poisoning deaths in the United States nearly doubled, from approximately 20 000 to 37 000, largely because of overdose deaths involving prescription opioid painkillers.1 This increase coincided with a nearly 4-fold increase in the use of prescription opioids nationally.2 In Washington, in 2006, the rate of poisoning involving opioid painkillers was significantly higher than the national rate.1 To better characterize the prescription opioids associated with these deaths and to reexamine previously published results indicating higher drug overdose rates in lower-income populations,3 health and human services agencies in Washington analyzed overdose deaths involving prescription opioids during 2004-2007. This report describes the results of that analysis, which found that 1668 persons died from prescription opioid-related overdoses during the period (6.4 deaths per 100 000 per year); 58.9% of decedents were male, the highest percentage of deaths (34.4%) was among persons aged 45-54 years, and 45.4% of deaths were among persons enrolled in Medicaid. The age-adjusted rate of death was 30.8 per 100 000 in the Medicaid-enrolled population, compared with 4.0 per 100 000 in the non-Medicaid population, an age-adjusted relative risk of 5.7. Methadone, oxycodone, and hydrocodone were involved in 64.0%, 22.9%, and 13.9% of deaths, respectively. These findings highlight the prominence of methadone in prescription opioid–related overdose deaths and indicate that the Medicaid population is at high risk. Efforts to minimize this risk should focus on assessing the patterns of opioid prescribing to Medicaid enrollees and intervening with Medicaid enrollees who appear to be misusing these drugs.