Introduction The ketogenic diet (KD) is increasingly used for the treatment of refractory epilepsy. The aim of this study was to evaluate possible adverse effects of the diet on cognition, behavior, psychosocial adjustment, and quality of life in school-aged children and adolescents. Method Fifteen subjects were assessed before diet initiation. After approximately 6 months, on diet treatment 11 patients (73%) were reassessed. We used a combination of individually administered psychological tests for the children and parent report questionnaires. Result Five of 15 patients had a seizure reduction of more than 50%. Cognition showed a small trend toward improvement in most patients. Psychosocial adjustment, on the other hand, showed small trends toward worsening. For mood, two areas showed a larger change, revealing more mood problems although this was not on a statistically significant level. Conclusion In this small group of children, there is no indication that the KD has a negative impact on cognition or social adaptation at short term. There is a tendency toward an increase in mood problems.