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A method is presented to construct age-reference curves for skinfolds and skinfold sums on longitudinal data, using the Box-Cox transformation: (xx - 1 )/X. The method consists of seven steps: 1. Search for X-values producing minimal skewness.2. Calculation of a fitting age curve of X; 3. Transformation of skinfold data at each age with the corresponding X-value and calculation of mean and standard deviation of the transformed data; 4. Drawing eye-fitted smoothed curves through these points; 5. Calculation of percentiles based on the transformed and smoothed means and standard deviations, and plotting these after transformation back to mm. 6. Comparison of percentages of observed measurements in the interpercentile ranges to the target ones; 7. Comparison of the obtained curves with the curves based on the raw data. With this method acceptable Gaussian distribution at each age could be obtained.