Diabetes type 1 seems to be more prevalent in epilepsy, and low-carbohydrate diets improve glycemic control in diabetes type 2, but data on the use of the classic ketogenic diet (KD) in epilepsy and diabetes are scarce. We present 15 months of follow-up of a 3 years and 6 months old girl with diabetes type 1 (on the KD), right-sided hemiparesis, and focal epilepsy due to a malformation of cortical development. Although epileptiform activity on electroencephalography (EEG) persisted (especially during sleep), clinically overt seizures have not been reported since the KD. An improved activity level and significant developmental achievements were noticed. Glycosylated hemoglobin (HbA1c) levels improved, and glycemic control was excellent, without severe side effects. Our experience indicates that diabetes does not preclude the use of the KD.