Research groups across Europe have been networking to share information and ideas about research on preschool children with autism. The paper describes preliminary work to develop capacity for future multi-site randomized controlled trials of early intervention, with a specific focus on the need to measure treatment adherence where parents deliver therapy. The paper includes a review of randomized and controlled studies of parent-mediated early intervention from two sources, a recent Cochrane Collaboration review and a mapping of European early intervention studies in autism published since 2002. The data extracted focused on methods for describing parent adherence, that is, how and to what extent parents carry out the strategies taught them by therapists. Less than half of the 32 studies reviewed included any measure of parent adherence. Only seven included a direct assessment method. The challenges of developing pan-European early intervention evaluation studies are discussed, including choice of intervention model and of important outcomes, the need for translation of measurement tools and achievement of joint training to reliability of assessors. Measurement of parent-child interaction style and of adherence to strategies taught need further study. © 2014 The Authors.