Gait analysis (GA) is widely used for clinical evaluations and it is recognized as a central element in the quantitative evaluation of gait, in the planning of treatments and in the pre vs. post intervention evaluations in children with Cerebral Palsy (CP). Otherwise, GA produces a large volume of data and there is the clinical need to provide also a quantitative measure of the patient's overall gait. Starting from this aim some global indexes were proposed by literature as a summary measure of the patient's gait, such as the Gait Deviation Index (GDI). While validity of the GDI is demonstrated for the evaluation of the functional limitation of CP patients, no studies have evaluated with the GDI the pre vs. post surgery gait condition in children with CP. The aim of our study was therefore to investigate the effectiveness of the GDI in the quantification of gait changes occurring after surgical intervention (gastrocnemius fascia lengthening for the correction of equinus foot deformity) in patients with CP. 19 children with CP were evaluated pre-operatively (PRE session) and about 1 year (POST: mean 13.1 [plus or minus] 5.1 months) after gastrocnemius fascia lengthening using 3D GA. From GA data the GDI was computed. The results evidenced that the GDI value in the PRE session was 70.4 [plus or minus] 14.8, showing a moderate global disturbance of the gait patterns of the patients. After the surgical treatment a significant improvement of the GDI mean value was found (82.9 + 7.4; p less than 0.05; CG [greater than or equal to] 100) with an improvement of 18% respect to the PRE session. A strong correlation (p = 0.83; p less than 0.05) existed between the GDI value in the PRE session and the percentage of improvement. Our results demonstrated that GDI seems to be an appropriate outcome measure for the evaluation of the effects of surgical treatment in CP. (Contains 1 figure.)