Evidence has accumulated that predictions of annual or half-annual growth rates are of limited validity when derived from extrapolations of short term growth rates. In order to investigate whether the poor predictive qualities of short sections of individual growth curves are caused by non-linearity of human growth, we have studied 73 healthy children, aged 2.9 to 15.9 years, with standard deviation scores for body height ranging between -3.3 and +3.0. The children were measured between 18 and 106 times once or twice per week throughout periods of 180 to 306 days by knemometry, a novel and noninvasive technique of accurate lower leg length measurement. Non-linearity of growth was found in about 70% of those children who could be measured more than 35 times. 45 children provided evidence for a characteristic up-and-down pattern of lower leg growth velocity consisting of sharp growth spurts ('mini growth spurts') alternating with periods of decreased growth velocity every 30 to 55 days. This pattern seemed to appear spontaneously, though in some cases we found a marked coincidence between periods of growth arrest and intermittent infectious illness.