Forty three infants were assigned randomly to a ready-to-feed infant formula or a standard formula that required reconstitution from powder. Despite similar nutrient composition of the two formulas those fed the powdered formula had significantly increased body weight and skinfold thickness gains, and became significantly heavier than a further group of 20 breast fed infants by 3 and 6 months. Of those fed the powdered formula 6/19 had become overweight (above the 90th or 97th centile) by 6 months, whereas 1/19 fed the ready-to-feed product was overweight at this age. While differences in fat absorption might have been contributory, our data suggest that errors in reconstitution of formula from powder might be the main cause for the growth differences observed. If it is appropriate to take the breast fed infant as a model, infants fed ready-to-feed formula in this study showed a more physiological pattern of growth than those fed a standard formula reconstituted from powder. These results require replication using other formulas as the findings have potentially important implications for infant feeding.