Treatment studies in child and adolescent psychiatry are increasingly characterized by long-term, multisite, randomized clinical trials (RCTs). During the course of these RCTs it is common for clinical exigencies to emerge that require rapid, direct intervention. The challenge is to provide clinically appropriate responses that do not contaminate the delivery, distinctness, and interpretation of the treatments under investigation. In multisite studies, the problem is compounded by the need to minimize cross-site differences in the delivery of adjunct treatments. Such minimization requires fully operationalized and manual-based procedures for clinically mandated intervention. The NIMH Collaborative Multisite Multimodal Treatment Study of Children With Attention-Deficit/Hyperactivity Disorder (ADHD)--"the MTA"--is a long-term multisite collaborative study in which children with ADHD were randomly assigned to either medication management, behavioral treatment, the combination, or community-comparison assessment and referral. In designing its study, the MTA developed a manual-based set of procedures (the MTA Adjunct Services and Attrition Prevention [ASAP] Manual) for situations not covered by the protocol treatments. The majority of cases requiring adjunct services fell into two major categories: (1) crisis/emergent situations and (2) imminent risk of attrition. This report describes the ASAP guidelines for dealing with cases that required adjunct services that the MTA Steering Committee adopted before initiating the trial. Although the manual-based guidelines are especially applicable to multisite RCTs, many of the procedures in the ASAP Manual can apply to any treatment study in children.