This study sought to evaluate the efficacy, tolerability, and safety of a ketogenic diet (KD) in the treatment of infantile spasms (IS), incorporating a revised protocol based on our previous experience with KD. We undertook a retrospective analysis of 43 children who suffered from catastrophic IS and tried KD from June 1995 to May 2004 in two Korean epilepsy centers. Outcome measures included seizure frequency, electroencephalograms (EEGs), adverse reactions, and neurological development. Overall, the diet achieved the seizure-free state in 53.5% (23/43) of patients and a greater than 90% reduction of seizure frequency in 62.8% (27/43) of patients. The seizure outcomes were highly concordant with improvements in EEG findings and development. In addition, KD could be maintained more safely and more comfortably because of our revised protocol that included an initial non-fasting diet, a short-term trial of 8 months, a more protein-rich diet with a lipid to non-lipid ratio of 3:1, and liquid ketogenic milk. Most complications were transient and KD was well tolerated in most cases. KD can be an effective alternative therapy for catastrophic IS, and additional benefits may be derived from constant revision of the diet in the future.