One of the problems in the care of healthy children is that mothers often give infant formulae to their normally growing infants in the first 4 months of life because they believe that they are unable to satisfy them. However, the association of fat content in breast milk with the sucking pattern of the infant is not clearly known. In order to determine whether the fat concentration of human milk was associated with sucking characteristics of the infants, 80 healthy 2-month-old babies and their mothers were included in the study. Milk creamatocrit was assessed at the beginning, at the first, fifth and 10th minutes and at the end of breast feeding. The creamatocrit values increased as the time elapsed from the beginning of breast feeding. There was an inverse relationship between milk volume and creamatocrit at both the beginning and the end of breast feeding (P < 0.05). The mean milk volume was higher and the mean creamatocrit at the end of breast feeding was lower in the infants whose 2-month weight gain was higher than the 75th percentile (75P) compared with those whose 2-month weight gain was lower than the 25th percentile (25P) (for milk volume 105 +/- 36 mL vs. 76 +/- 30 mL, respectively, P < 0.05; and for creamatocrit value 11.4 +/- 4.0 vs. 14.3 +/- 3.8, respectively, P < 0.05). Milk volume is the most important factor affecting weight gain of exclusively breast-fed babies, and fat concentration of human milk is not a primary determining factor in individual differences in weight gain of the infants and appears to be secondary to factors such as number of feeds per day, duration of breast feeding and the volume of milk sucked.