Weight control programs for obese children monitor change in body mass index (BMI) adjusted for age. However, change can be measured in several ways: raw (kg/m2) units, percentage, z-scores or centiles. The suitability of the different measures is not known. To identify the optimal BMI measure for change, whose short-term variability is most consistent for children across the spectrum of adiposity. An Italian kindergarten. A total of 135 (66 female) children aged 29-68 months at baseline, with BMI measured three times over a 9-month period. Each child's short-term variability in adiposity was summarized by the standard deviation (s.d.) of BMI and BMI % adjusted for age, and BMI z-score and BMI centile. The s.d.'s were then compared in obese and nonobese children, and also correlated with each child's baseline BMI z-score. The within-child s.d.s of BMI z-score and BMI centile were significantly smaller in obese than nonobese children, while the s.d.s of BMI and BMI % were similar in the two groups. Also, the within-child s.d.s of z-score and centile, and to a lesser extent BMI %, were significantly inversely correlated with baseline z-score, whereas the s.d. of BMI was not. The changes in adiposity over time, as assessed by the four measures, were very highly correlated with each other, particularly for BMI with BMI %. Even though BMI z-score is optimal for assessing adiposity on a single occasion, it is not necessarily the best scale for measuring change in adiposity, as the within-child variability over time depends on the child's level of adiposity. Better alternatives are BMI itself or BMI %. Our results underscore the importance of using a relatively stable method to assess adiposity change when following children at risk of obesity.