The Index of Complexity, Outcome and Need (ICON), based on international opinion, has been proposed as a multipurpose occlusal index. The aim of this study was to validate the ICON for treatment need in the Netherlands by relating it to Dutch orthodontic opinion. Furthermore, the reliability of this index was explored, for both a calibrated orthodontist and non-calibrated orthodontists. A sample of 102 patients was chosen which represented the actual distribution of severity of malocclusion experienced by orthodontists in every day practice. The ICON was scored, based on complete patients records of those 102 patients, by an examiner calibrated in the use of this index. The results were compared with the opinion about treatment need of seven Dutch orthodontists – the ‘gold standard’. Nine non-calibrated orthodontists also scored the ICON for 49 patients. The intra-examiner agreement of both the non-calibrated and the calibrated orthodontists was moderate to high [0.52–0.86 and 0.89, respectively, measured with the Intraclass Correlation Coefficient (ICC)]. The inter-examiner agreement of the ICON score of the nine orthodontists was moderate measured with the single estimate of the ICC (0.60), and high measured with the average estimate (0.93). Spearman's correlation coefficient between the ICON score (calibrated) and the gold standard was sufficient: 0.78. The sensitivity and specificity were 1 and 0.36, respectively. The best compromise between sensitivity and specificity was at a cut-off point of 52, instead of the international ICON cut-off point of 43. There was a significant difference in ICON score between the non-calibrated orthodontists and the calibrated orthodontist, mainly based on the Aesthetic Component (AC) of the Index of Orthodontic Treatment Need (IOTN). It can be concluded that the ICON needs to be adjusted when used to determine treatment need in the Dutch orthodontic population.