PURPOSE:Infantile spasm is the most important epilepsy syndrome that brings catastrophic results in childhood. Persistent spasms and hypsarrhythmia have been known to regress the brain maturation and development. Therefore, it is very important in these patients to find a way to achieve developmental progress as good as possible. The objective of this study was to compare the influence of various etiology on developemental outcome and to determine which therapy has a more favorable development outcome. METHODS:We reviewed 95 children diagnosed as infantile spasm between 1991 and 2005 at College of Medicine of Yonsei University and Sang-gye Paik Hospital. We compared possible factors to predict the developmental outcomes in terms of patient characteristics, etiology, seizure duration and seizure outcomes along with various treatment modalities such as antiepileptic drugs, steroid, ketogenic diet, and surgery. RESULTS:Mental retardation occured in 81(85.3%) of the patients with infantile spasms and 38(40.0%) suffered from propound mental retardation. In predicting the developmental outcomes, the most important factor was found to be the etiology. While only 13(56.5%) patients with cryptogenic infantile spasms had severe to profound mental retardation, 53(73.6%) patients with symptomatic etiologies did. Other factors shown to be associated with a good progress included high developmental scores at the onset, a short duration of spasms, an early effective control of spasms, early consideration of non-pharmacologic treatment such as ketogenic diet and surgery when the seizures filed to respond to antiepileptic drugs. CONCLUSION:This retrospective review suggests that it is possible to improve the developmental outcomes of infantile spasms by making correct etiologic diagnosis and providing early appropriate therapy chosen from variable treatment modalities.