Objective. To identify the factors associated with drug abuse treatment center participation in interorganizational relationships (IORs). Data Sources. Three nationally representative samples of outpatient drug abuse treatment units surveyed in 1990, 1995, and 1999/2000 as part of the National Drug Abuse Treatment System Survey (NDATSS), stratified by public/private status, treatment modality (methadone or nonmethadone), and organizational affiliation. Study Design. Probit analyses on 647 lagged treatment center-year observations from the years 1990 to 1995 with outcomes in 1995 and 2000, respectively. Standard errors were adjusted for clustering of center-year observations within centers. Principal Findings. Centers with greater motivation to form IORs (e.g., as a result of client diversity or government revenue) were more likely to do so, as were centers with greater opportunities to form IORs (e.g., centers whose directors participated in policy making). Conclusions. Both motivating and enabling factors promoted the formation of IORs by drug abuse treatment centers. Managed care also played a distinct role, in this case appearing to undermine interorganizational cooperation. Because IORs can improve access to care and quality, policy makers should consider using both incentives and support such as management training to promote IOR formation. © Health Research and Educational Trust Organizational.