Purpose: Ketogenic diet therapy (KD) has been used to treat children with refractory generalized epilepsy. We herein reported the efficacy of KD for West syndrome (WS) resistant to ACTH therapy. Subjects: Subjects, consisting of 6 patients (3 boys, 3 girls) with WS who continued to have epileptic spasms (ES) and hypsarrhythmia, received KD because other treatments including ACTH therapy failed to control WS. Methods: We retrospectively studied the clinical details of these patients and the efficacy of KD. Results: The mean age at the onset of epilepsy was 4months (0–15months). The underlying etiology consisted of lissencephaly, Down’s syndrome, and focal cortical dysplasia. Hypsarrhythmia disappeared 1month after the introduction of KD in 5 patients. The disappearance of ES was achieved in 2 patients, the frequency of ES episodes was 80% less in 3, and no change was observed in 1. Psychomotor development was promoted in 5 patients, along with improvements in ES and EEG. Gastrointestinal complications and lethargy, presumably caused by rapid ketosis, were reported as side effects in 3 patients during the first week of KD. Side effects including lethargy, anorexia, and unfavorable weight gain continued thereafter in these patients in spite of tolerance to KD. Conclusion: KD was effective for WS resistant to ACTH therapy, although gastrointestinal side effects should be considered when introducing KD to milk-fed infants.