In the 1960s, educational psychologist Marjorie Boxall developed Nurture Groups (NGs) in response to the growing number of children who were deprived of healthy nurturance in early life and who, as a result, were failing to cope with the demands of school. To date, research on this intervention model has consistently shown that students who attend a NG for at least one school year are much more likely to demonstrate improvements in school functioning than other at-risk students who do not. However, the conclusions that can be drawn about the beneficial effects of NGs are somewhat limited by the heterogeneity in practices among groups bearing the NG name and by the absence of data explicitly linking positive student outcomes to specific practices within NG classrooms. Both these limitations could be addressed by a more systematic effort to consider the question of implementation fidelity. Thus, the objective of the present study was to measure student progress in NGs for which detailed information about the intervention's implementation fidelity was available. In two NGs known to implement relatively high frequencies of nurture-based interventions (i.e., attunement strategies and constructive behaviour support), results revealed statistically and clinically significant improvements in social, emotional and behavioural functioning following a nine-month period of intervention. More research relating NG efficacy to implementation procedures is needed in order to better understand the most effective ingredients of this intervention.