Abstract: This paper describes a three-year research project with CORE-OM (Clinical Outcome Routine Evaluation – Outcome Measure) and illustrates the results of the evaluation of the clinical outcome of psychotherapy in clients undergoing psychotherapeutic process with the Gestalt method within the National Organisation of Gestalt Therapy (NOGT), and the Italian Society of Gestalt Psychotherapy (SIPG). The data was gathered using CORE-OM. The study is an example of a practice-oriented research in Gestalt therapy because it introduces a widely used quantitative method to evaluate the clinical outcome of psychotherapy (CORE-OM) as an integral part of clinical work in a relational approach. The authors have repeated within the Italian Gestalt community a similar study conducted in the UK in 2011 and compared both findings. Results indicate that Gestalt therapy has been effective in improving the condition of clients, with an effect size comparable to those that were observed in similar studies conducted in other countries and with different modalities. Keywords: Gestalt therapy, treatment effectiveness, outcome research, CORE-OM, long- term psychotherapy.