To characterize IgE levels at birth and changes in those levels during the first year of life and to identify Factors that might influence IgE levels in infancy, we measured IgE levels in 1074 umbilical cord sera and in 697 sera obtained at 9 months of age in a healthy population of infants enrolled at birth into the Children's Respiratory Study in Tucson, Arizona, U.S.A. Serum IgE levels at birth and 9 months were log normally distributed with geometric means of 0.09 and 3.87 IU/ml, respectively. Cord serum IgE levels were unaffected by maternal smoking. Levels varied according to month of birth with a nadir in September. Cord and 9-month serum IgE levels were higher in boys than in girls, Hispanics compared with Anglos, and infants who developed eczema compared with those who did not, but the mean increases in log IgE from birth to 9 months were not significantly affected by these factors. A significant correlation between IgE levels at cord and 9 months was observed (r= 0.44; P < 0.0001). Also, mean log IgE levels at 9 months in infants grouped according to cord serum IgE levels maintained the same rank order of mean values as the cord groups. These data indicate that 9-month IgE levels are influenced by cord serum IgE levels and that the main influence of gender, ethnicity and susceptibility to eczema on IgE levels occurs before birth.