OBJECTIVE To describe the processes and costs of engaging practice nurses (PNs) to establish a cluster randomised controlled trial (RCT) to study type 2 diabetes in general practice. DESIGN, SETTING AND PARTICIPANTS Descriptive study of the processes and costs of engaging PNs from 59 general practices in Victoria that were participating in the Patient Engagement And Coaching for Health (PEACH) study, prior to practices being randomly assigned in the cluster RCT. MAIN OUTCOME MEASURES Estimated direct research costs and personnel costs for establishing a general practice-based research project involving PNs (eg, costs for approaching Victorian Divisions of General Practice and the Australian Practice Nurses Association; practice and patient recruitment; research project establishment at general practices; and PNs' training, support and engagement during the study establishment period). RESULTS The estimated cost to establish our PN-led general practice-based cluster RCT was over $110 000, with an average cost of $2000 per practice. Direct research and personnel costs were considerably higher than anticipated. Lack of research skills among PNs required intensive hands-on support from the research team. CONCLUSIONS It is feasible to undertake a PN-led, general practice-based clinical trial in diabetes care. Future research funding needs to account for recruitment costs, including the need to build PN research capacity, and to overcome the inherent difficulties of engaging practices in complex intervention trials in primary care. TRIAL REGISTRATION International Standard Randomised Controlled Trial Number Register ISRCTN50662837.