To compare the morbidity between the breast-fed and artificial formula-fed Infants, the frequency of diseases during infancy was studied. The subjects were 37 breast-fed infants and 41 formula-fed infants aged 12-15 months. The data were obtained while they visited the pediatric out-patient clinics. The results were as follows : 1. Most prevalent diseases in the infancy were respiratory and gastrointestinal illnesses. 2. During the first 6 months the morbidity was significantly lower in the breast-fed than in the formula-fed 3. The frequency of respiratory and gastrointestinal diseases was significantly higher in the formula-fed than in the breast-fed infant during the first 6 months.