Purpose: The ketogenic diet (KD) is increasingly used for the treatment of refractory epilepsy in childhood because of the beneficial effect on seizure reduction. The aim of the current study was to objectively assess cognition and aspects of behavior during the first 4months of a randomized controlled study in children and adolescents. Methods: Participants from a tertiary epilepsy center were randomized to a KD group (intervention) or a care-as-usual (CAU) group (control). Follow-up assessments on cognition and behavior were performed approximately 4months after initiation of the KD with a combination of parent report questionnaires and individually administered psychological tests for the children. Results: A total of 50 patients were enrolled in this study, 28 patients from the KD group and 22 patients from the CAU group. The KD group showed lower levels of anxious and mood-disturbed behavior and was rated as more productive. Cognitive test results showed an improvement of activation in the KD group. Conclusions: This study showed a positive impact of the KD on behavioral and cognitive functioning in children and adolescents with refractory epilepsy. More specifically, an activated mood and cognitive activation were observed in patients treated with the KD.