The serum concentration of apo(a), the unique apolipoprotein of lipoprotein (a), reflects serum lipoprotein (a) levels. High concentrations are associated with increased cardiovascular risk. Inasmuch as atherogenesis may begin in childhood, the early expression of the apo(a) gene and relationships between serum levels in infants and their parents were explored. Serum apo(a) and lipid profiles were measured in 51 infants when aged 8.5 +/- 2 months. They were from among 1032 consecutively born babies in whom apo(a) levels had been measured on day 2 to 5. Levels in 18 infants were in the top 5% of the neonatal apo(a) distribution and in 33 from below the 95th percentile. Parental values were also assessed. Infants' apo(a) levels (n = 51) at the ages of 2 to 5 days and 8.5 +/- 2.3 months were highly correlated (r = .73, P less than .0001) and increased from an initial median value of 48 U/L (range 1 to 462 U/L) to 100 U/L (5 to 969 U/L) at 8.5 months, and they were then not different from parental levels. Measurements at both times were closely correlated with parental levels. Regression coefficients between 8.5-month levels, and the levels of fathers, of mothers, and the average level of both parents were 0.439, 0.521 and 0.93, respectively (P less than .0001 for each). It is concluded that the gene for the regulation of apo(a) is fully expressed before the age of 1 year. The apo(a) levels in infants during this time track closely and are predictive of parental values.(ABSTRACT TRUNCATED AT 250 WORDS)