OBJECTIVE This paper is a description of a method to develop and conduct a customized psycho-educational intervention fidelity assessment as part of pilot work for an efficacy study. A tool designed to assess treatment fidelity to a psycho-education intervention for patients with end-stage renal disease and their surrogate decision makers, Sharing the Patient's Illness Representations to Increase Trust, is presented as an illustration. BACKGROUND Despite the specificity and idiosyncrasy of individual interventions and the call to systematically evaluate treatment fidelity, how to accomplish this goal has not been clarified. Tools to adequately measure treatment fidelity are lacking. METHODS We developed the Sharing the Patient's Illness Representations to Increase Trust Treatment Fidelity Assessment tool by identifying elements that were idiosyncratic to the intervention and those that could be adapted from existing tools. The tool has four components: overall adherence to the intervention content elements; pacing of the intervention delivery; overall dyad responsiveness; and, overall quality index of intervention delivery. The study was undertaken between 2006 and 2008. RESULTS Inter-rater reliability ranged from 0.80 to 0.87 for the four components. The tool showed utility in training and monitoring, such as detecting unplanned content elements delivered and the use of proscribed communication behaviours. CONCLUSIONS Psycho-educational interventions are one of the most common types of nursing interventions worldwide. Use of fidelity assessment tools customized to the individual interventions may enhance systematic evaluation of training and monitoring treatment fidelity.