BACKGROUND: The incidence of epilepsy is greatest in the first 2 years of life, an age group where there is generally a poor prognosis for both seizure control and neurodevelopmental outcome. Early control of seizures can be associated with better developmental outcome but many of the epilepsies presenting in infancy are poorly responsive to antiepileptic medication. The ketogenic diet (KD) is a high-fat, low-carbohydrate diet designed to mimic the effects of starvation on the body. Dietary fat is converted into ketones in the body and used as an energy source by the brain. The KD has been shown to be successful in controlling seizures in many observational studies, and in two randomised controlled trials (RCTs) in older children. However, little evidence is available in the very young. METHODS/DESIGN: An open-label RCT where eligible children (age 3 months to 2 years with epilepsy who have failed two antiepileptic drugs (AEDs)) undergo baseline assessment, including medical and seizure history. Participants then start an observation period (7 or 14 days) with documentation of seizure frequency. Randomisation will occur on day 8 or day 15 to receive the KD or a further AED; the allocated treatment will commence on day 15, with instruction and training. A second assessment (4 weeks after start of treatment) will include a clinical review and tolerability questionnaire (modified Hague Scale of Side Effects - for those allocated to the KD group). Assessments will be repeated at 8 weeks after the start of treatment including biochemical investigations, after which, according to patient response, KD (diet group) or AED (standard AED group) will then be continued or changed. Those in the AED group who have failed to achieve seizure control at the 8-week assessment will then be offered KD outside the context of the trial. Those in the KD arm who fail to achieve seizure control will be changed to standard clinical management. All patients will be followed up for 12 months from randomisation for retention, seizure outcome, quality of life and neurodevelopmental status. DISCUSSION: The slow rate of recruitment is an ongoing practical issue. There is a limitation to the number of eligible patients compared to what was predicted, mainly due to the nature of this patient group. After a substantial amendment to widen inclusion criteria and reduce the baseline period to 7 days for patients with a high seizure burden, the rate of recruitment steadily increased. A number of operational concerns regarding dietetic time were also highlighted impacting on the recruitment rate. However, the combination of a low dropout rate and the opening of further centres, the trial should successfully meet the final recruitment target. All nine centres are now recruiting and we hope to open further centres within the UK. TRIAL REGISTRATION: ClinicalTrials.gov, identifier: NCT02205931 . Registered on 16 December 2013.