Contingent staffing arrangements are defined as conditional and transitory work arrangements. In the drug abuse treatment sector, contingent staffing arrangements have the potential to improve treatment if they are used to increase access to needed services. Alternatively, such arrangements could interfere with the development of consistent, long-term client-staff relationships. Unfortunately, little is known about the consequences of or influences on contingent staff arrangements in this sector. The goal of this study is to examine the conditions under which outpatient substance abuse treatment organizations are more likely to use contingent staffing arrangements. Building on previous research on the social organization of health care structures and practices, we develop a conceptual model based in market economics and institutional perspectives to suggest that treatment organizations choose contingent arrangements in response to market conditions and uncertainty, institutional demands, and client needs. Using data from a nationally representative study conducted in 1988, 1990, and 1995, we find limited evidence that drug treatment units use contingent staff in response to market pressures. Labor market and demand uncertainty, however, are systematically associated with greater use of contingent staff. Study results suggest that expectations and norms from the institutional environment, particularly the organizational context of the treatment unit are strong predictors of the use of contingent staff. By considering both market and social influences of contingent staffing, we contribute to a growing body of research on how markets and institutions interact to influence organizational structures and practices in the health care system.