Drawing from an institutional-theory perspective on innovations in organizations, this paper examines the use of human immunodeficiency virus (HIV) prevention practices by the nation's outpatient substance abuse treatment units during a critical period from 1988 to 1995. An institutional perspective argues that organizations adopt new practices not only for technical reasons, but also because external actors actively promote or model the use of particular practices. We examine the extent to which treatment units use several practices to prevent HIV infection among their clients and among drug-users not in treatment. Results from random-effects regression analyses of national survey data show that treatment units significantly increased their use of HIV prevention practices from 1988 to 1995. Further, the results show that treatment units' use of prevention practices was related to clients' risk for HIV infection, unit resources available to support these practices, and organizational support for the practices. Implications are discussed for an institutional view of organizational innovation as well as for research on HIV prevention.