The weight increment profiles of 20 low birthweight babies measured during the first two months of extrauterine life were analysed. The babies were weighed daily, and the weight profiles showed minor irregularities when compared with an interpolated linear trend. When increments were plotted at two week intervals a linear increase in weight velocity was seen, but when increments were computed every three days, the velocity profile was non-linear and pulsatile. All cases studied showed regular pulsatile patterns of weight velocity during the first two months of life. A mean profile of the 20 babies permitted estimation of the periodicity of the pulsing: the cycle alternated every nine to 11 days. A non-linear pattern was found in the published series of unsmoothed data that have been widely adopted as standards for growth in low birthweight babies.