The availability and diversion of prescription-type opioids increased dramatically in the first decade of the twenty-first century. One possible consequence of increased prescription opioid use and accessibility is the associated rise in opioid dependence, potentially resulting in heroin addiction. This study aimed to determine how common initial dependence on prescription-type opioids is among heroin injectors; associations with demographic and drug-using characteristics were also examined. Interview data were collected at syringe exchanges in King County, Washington in 2009. Among the respondents who had used heroin in the prior four months, 39% reported being "hooked on" prescription-type opioids first. Regression analysis indicated that younger age, sedative use and no recent crack use were independently associated with self-report of being hooked on prescription-type opioids prior to using heroin. These data quantify the phenomenon of being hooked on prescription-type opioids prior to initiating heroin use. Further research is needed to characterize the epidemiology, etiology and trajectory of prescription-type opioid and heroin use in the context of continuing widespread availability of prescription-type opioids.