To obtain age references for sitting height (SH), leg length (LL), and SH/H ratio in the Netherlands; to evaluate how SH standard deviation score (SDS), LL SDS, SH/H SDS, and SH/LL SDS are related to height SDS; and to study the usefulness of height corrected SH/H cut-off lines to detect Marfan syndrome and hypochondroplasia. Methods: Crosssectional data on height and sitting height were collected from 14 500 children of Dutch origin in the age range 0- 21 years. Reference SD charts were constructed by the LMS method. Correlations were analysed in three age groups. SH/H data from patients with Marfan syndrome and genetically confirmed hypochondroplasia were compared with height corrected SH/H references. Results: A positive association was observed between H SDS, SH SDS, and LL SDS in all age groups. There was a negative correlation between SH/H SDS and height SDS. In short children with a height SDS <- 2 SDS, a cut-off limit of + 2.5 SD leads to a more acceptable percentage of false positive results. In exceptionally tall children, a cut-off limit of - 2.2SDS can be used. Alternatively, a nomogram of SH/H SDS versus H SDS can be helpful. The sensitivity of the height corrected cut-off lines for hypochondroplasia was 80% and for Marfan syndrome only 30% . Conclusions: In exceptionally short or tall children, the dependency of the SH/H ratio (SDS) on height SDS has to be taken into consideration in the evaluation of body proportions. The sensitivity of the cut-off lines for hypochondroplasia is fair. To obtain the SH standard deviation score (SDS), LL SDS, SH / H SDS, and SH / LL SDS are related to height SDS; and to study the usefulness of height corrected SH / H cut-off lines to detect Marfan syndrome and hypochondroplasia. Methods: Crosssectional data on height and sitting height were collected from 14 500 children of Dutch origin in the age range Reference SD charts were constructed by the LMS method. Correlations were analyzed in three age groups. SH / H data from patients with Marfan syndrome and genetically confirmed hypochondroplasia were compared with height corrected SH / H references. Results: A positive There was a negative correlation between SH / H SDS and height SDS. In short children with a height SDS <- 2 SDS, a cut-off limit of + 2.5 SD leads to a more acceptable percentage of false p The sensitivity of the height corrected cut-off lines for hypochondroplasia was 80% and for Marfan syndrome only 30%. Conclusions: In exceptionally short or tall children, the dependency of the SH / H ratio (SDS) on height SDS has to be taken into consideration in the evaluation of body proportions. The sensitivity of the cut-off lines for hypochondroplasia is fair.