In this article, managers' perceptions of risk on entering a newly formed primary health care partnership are explored, as are the mechanisms of trust and control used to manage them. The article reports a qualitative component of a 2-year National Health and Medical Research Council funded study of trust within the structures of a Primary Care Partnership (PCP) in Victoria, Australia. Multiple methods of data collection were employed. We found that managers identified risks at system, partnership and agency levels, and that as trust was built, concern about risks diminished. Trust effectively facilitated joint action, but it was betrayed on occasions, in which case the informal power of group process was used to contain the problems. The implications of this study for policy makers are in terms of understanding how perceptions of risk are constructed, the ways managers use social control to create a safer context in which to locate the trust-based relationships that facilitate joint action, and the importance of institutional arrangements. Without trust, joint action is hard to achieve, and without control, it is difficult to prevent breaches of trust from inhibiting joint action.