Background: Because microflora has been reported to have an important effect on the development of allergic disorders, we measured intestinal microflora levels in 4-month-old infants and studied the development of allergic disorders. Methods: Blood samples from 18 4-month-old infants and 15 1-year-old infants were examined for total serum IgE and specific IgE antibodies. Stool samples from 18 4-month-old infants were examined for the presence of microflora. Results: A positive correlation was observed between the ratio of breast-feeding at 1 month and the percentage of bifidobacteria in the intestine at 4 months (correlation ratio = 0.54; P = 0.022). Atopic dermatitis was observed in 12 of 18 infants at 4 months and in five of 15 infants at 1 year. Egg white-specific IgE was positive (≥0.70 UA/mL) in six infants at 4 months and in seven infants at 1 year. No relationship was observed between the percentage of bifidobacteria, lactobacilli or clostridia in the intestinal tract at 4 months and the development of allergy. However, all five infants who exhibited a percentage of bacter- oides (compared with the total intestinal microflora level) of more than 10% at 4 months had positive egg white-specific IgE and higher levels of total IgE (>25 IU/mL) at 1 year; these relationships were statistically significant (P = 0.01). Conclusions: Colonization with bacteroides at 4 months of age is suggested to be related to the allergic state at 1 year of age.