A wide variety of organizations has become involved in providing medical and social services to people living with human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS). Although there is much interest among policymakers, service providers, and clients in coordination among HIV/AIDS service organizations, few studies have used network analytic tools to examine existing systems of HIV-related care. In an effort to fill this gap, this study used network analysis methods to describe several aspects of the interorganizational relationships among 30 HIV/AIDS service agencies in Baltimore, Maryland. Client referrals to other organizations, client referrals from other organizations, exchange of information about shared clients, formal written linkage agreements for client referrals, and joint programs were each examined as a distinct type of network tie, with each the basis of a separate network among these 30 organizations. All of the networks except the one based on joint programs were relatively well connected, with most organizations either directly or indirectly linked. Most of the interorganizational collaboration occurred on a rather ad hoc basis for the purposes of meeting the more immediate needs presented by clients. Highly structured coordination involving substantial investment of resources and ongoing interagency activities appeared to be less common. The findings from this study also suggest that the providers in Baltimore tend to work directly with others as client needs arise rather than negotiating through "clearinghouse" types of organizations. Of the 30 HIV/AIDS service organizations, 5 were highly central in at least four of the five different types of networks. These five organizations--each having a critical role in the continuum of care--may be considered the most central core of the HIV/AIDS service delivery network in Baltimore. These organizations tend to be those that have been created specifically to provide HIV-related services or that specialize in HIV/AIDS care. This research can help policymakers understand how an HIV-related service delivery network may function and delineate key features of a network. In all communities, this type of assessment is critical to designing interventions to promote collaboration that are feasible within the context of existing interorganizational relationships. This type of data also has implications for informing activities to build the capacity of HIV/AIDS service organizations.