Serum IgE levels were followed throughout the first year of life in a sample of 34 infants from atopic and nonatopic families. Their parents' serum IgE levels were also measured. Neonatal levels ranged from 0 to 10 U/ml. Half of the study group, which included a high proportion of infants from atopic families, maintained serum IgE levels below 10 U/ml until 1 yr of age. In the remainder, the serum IgE exceeded 10 U/ml before 1 yr of age, but the time of increase and the rate of increase varied from infant to infant; the sooner the IgE started to increase, the higher was the serum IgE level at 1 yr of age. The relationship of serum IgE level to the development of atopic disease and to other parameters was analyzed. Elevation of serum IgE level at or before 1 yr of age was highly correlated with atopic disease in the first 2 yr of life. The elevation preceded the manifestation of atopy.