Our primary aim in this study has been to evaluate selected conditions thought to influence the entry of injecting drug users (IDU) into detoxification and methadone maintenance programs, making use of a prospective study design to strengthen a cross-sectional investigation of these conditions. To begin our investigation, we analyzed cross-sectionally gathered data on 2879 IDUs who had been recruited through extensive community outreach efforts and who were interviewed at the time of recruitment. We then tested our hypotheses about suspected determinants of entry into treatment by analyzing prospectively gathered data on 1039 active drug users who had no recent history of being treated for drug problems. Among these 1039 IDUs (a subset of the initial cross-sectional sample), 144 entered a detoxification program between their recruitment interview and their next follow-up interview, conducted about six months after recruitment (range: 3.5-9.5 months) and 64 entered a methadone maintenance program during that observation interval. Using multiple logistic regression analyses, we found that a recent drug overdose, relatively higher frequency of injecting drugs, and a history of prior arrest or treatment were independent predictors of entry into detoxification. Being married or living with a partner, being female, a lengthy duration of drug use (> 10 years), and a history of prior treatment were independent predictors of entry into methadone maintenance. These findings shed light on what appears to be a different profile of suspected determinants of entry into a detoxification treatment versus methadone maintenance treatment, and help to clarify some potential differences between treated and untreated drug users that ought to be considered when evaluating results of investigations with IDU participants recruited solely from treatment settings.