The aim of this study was to establish reference values and factors associated with serum AFP elevation in infants. Five hundred twenty-four samples collected from infants up to the age of 2 years at the University Hospital Düsseldorf (Germany) were analyzed. At birth mean serum AFP levels were 41,687 ng/ml in 256 term babies and 158,125 ng/ml in 90 premature babies born before the 37th gestational week, excluding samples from children with factors known to be associated with AFP elevation. In the first 4 weeks of life, AFP levels decreased by 50% in 5.1 days in term babies. Between day 180 and 720 of life, AFP levels up to 87 ng/ml were within the 95.5% interval (assumed logarithmic normal distribution) with a mean of 8 ng/ml without a further decline. By the age of 2 years the infants of this study had not reached adult serum AFP levels (0-6 ng/ml).