Persistent infant crying and "colic" have been linked in some studies to feeding, but this association has not been tested in a planned longitudinal study comparing breast- with formula fed babies. We used validated maternal diaries of infant behaviours, kept for three days at both two and six weeks of infant age, in a comparative study of 97 breast- or formula fed babies. The total duration of overall crying rose significantly between 2 and 6 weeks in breast-fed infants and fell in those fed formula. At 6 weeks, breast-fed infants cried an average of almost 40 minutes more per day than formula fed infants; and 31% cried for more than three hours per day, compared with only 12% of the formula fed group. At six weeks, breast-fed infants also slept almost 80 minutes less per day than the formula fed babies. While six weeks is the established peak age for infant crying, those fed formula peaked much earlier and at 2 weeks intense crying/colic behaviour occurred in 43% of formula fed babies and just 16% of those fed by breast. These findings link the timing of the infant crying peak to the mode of feeding. Our data indicate that any regimen designed to reduce crying should commence in the neonatal period in formula fed infants.