BACKGROUND: Growth and development of infants has been an important topic in pediatrics for a long time. Infants must be provided with food containing all necessary nutrients. Breast milk is believed to be the most desirable natural and cheapest food for well-balanced nutrition. But with the progress in the development of substitute food in developed countries, it is thought that formula milk can meet the requirement for infant growth. During early infancy, growth, as the most sensitive index of health, is therefore a critical component in evaluating the adequacy of breast-feeding, mixed-feeding and formula feeding. Iron status is another important index of infant health. Iron deficiency anemia remains the most prevalent nutritional deficiency index in infants worldwide. This study is to compare infants in Beijing at 4 months who are on three different feeding modes (breast feeding, mixed feeding and formula feeding) in physical changes and iron status. The results may provide new mothers with support in feeding mode selection, which will also be helpful to the China Nutrition Association in feeding mode education. METHODS: This is a cohort study. One thousand and one normal Beijing infants were followed regularly for 12 months. Body weight and horizontal length were measured. Hemoglobin, red blood cell counts, mean corpuscular volume, mean corpuscular hemoglobin and serum iron were analyzed at 4 months. RESULTS: The breast feeding percentage in the first 4 months was 47.9%. The feeding mode was not significantly related to maternal delivery age, education, labor pathway nor infant sex (P>0.05). Infant boys and girls exclusively breast-fed from 0 to 4 months had the highest weight at 0-6 months. The anemia rate of breast-fed infant boys at 4 months was the highest. CONCLUSIONS: Breast feeding should be given more emphasis. It is compulsory for new mothers to breast-feed their infants if possible. Social environment should also guarantee the requirement for breast feeding. Furthermore the normal values of hemoglobin, mean corpuscular volume and serum iron, which were originally used to judge children's iron deficiency anemia, might not be optimal for evaluating infants. There might be a need to develop sex-specific cutoff levels of hemoglobin, mean corpuscular volume and serum iron for infants.