Clinical pictures of 21 cases with I-cell disease patients, 12 males and 9 females, were analyzed. Characteristic coarse facial features and shortness of stature were observed in all cases. In general, the motor development was found to be more severely retarded than the mental development of the patients. Rather little involvement of the nervous system seemed to cause somewhat acceptable mental development in some cases, and also cause the absence of epileptic seizures in all cases. Involvement of the cardiovascular system, especially progressive hypertrophic cardiomyopathy, could be highly responsible for frequent sudden death of I-cell disease patients.