Plasma cholecystokinin concentrations were measured with radioimmunoassay in breast fed infants in Sweden (n = 101) and formula fed infants in Italy (n = 119). Blood samples were collected from the infants during the first five postnatal days, just before feeding in a cross-sectional way. We found no significant difference in the hormone concentration in relation to age, in breast fed or in formula fed infants. Breast fed infants had significantly higher plasma hormone concentrations than formula fed infants during the postnatal period 0-5 days: 2.8 pmol/l (1-10 pmol/l), median (range) versus 2.2 pmol/l (1-25 pmol/l), (P < 0.02). It cannot be excluded that the results were influenced by the circumstances that the samples were collected from two separate populations. However, the higher hormone levels in breast fed infants may also be the expression of the fact that natural feeding per se promotes cholecystokinin secretion in newborn infants.