The aim of this work was to determine the usefulness of the Index of Orthodontic Treatment Need (IOTN) in epidemiological studies undertaken to assess orthodontic treatment need of a given population, as well as in the process of defining indications to orthodontic treatment in individual cases with regard to objective treatment need and expressed demand for treatment. The correlation of the Aesthetic Component of IOTN (AC degree) with demand for treatment, usefulness of IOTN for screening studies, necessary modifications to make IOTN more acceptable to Polish orthodontists, and value of IOTN as a diagnostic and measuring tool for the assessment of orthodontic treatment need were addressed. The correlation between AC and demand for treatment, as well as the usefulness of IOTN in epidemiological studies were investigated with questionnaires and clinical examination using both components of IOTN (Figs. 1, 2, 3). The study group consisted of 264 schoolchildren aged 12 years. The results show that poor dental aesthetics is the main motivating factor to undertake orthodontic treatment (Tab. 1) and that demand for orthodontic treatment exceeds treatment need determined by IOTN (Tab. 2, 3). A regular distribution of AC degrees relative to the demand for treatment was obtained (Fig. 4) revealing the necessity to modify treatment need categories of this component in order to identify individuals with significant subjective treatment need. The modified aesthetic component would comprise the following treatment need categories: Grades (photographs) 1-2--"no demand for treatment"; 3-4--"borderline demand for treatment"; 5-10--"great demand for treatment". Excellent reproducibility of the Index (high values according to Kappa statistics), its universality, usefulness for various purposes, and simplicity in practice make the Index valuable for screening studies and assessment of treatment need in a given population. In order to test the reliability of the Dental Health Component of IOTN (the degree of conformity of this component with treatment tendencies among orthodontists), DHC criteria and subjective evaluation of treatment need were compared. This part of the study was carried out in a group of six orthodontists. 50 pairs of plaster casts covering a wide range of occlusal anomalies were analyzed. The subjective need for orthodontic treatment was stated for each case taking into account treatment priority and occlusal features determining treatment need. The results reveal a moderate conformity of DHC criteria with the orthodontist's opinion (Tab. 4 A, B) and a tendency among orthodontists to qualify for treatment also cases without treatment need according to IOTN recommendations. The discrepancy between the orthodontist's assessment of treatment need and DHC criteria applies mainly to cases with reverse overjet, front open bite, front or total crossbite and crowding in buccal segments of dental arches (Tab. 4 A, B). Matching the dental health component with opinions of Polish orthodontists requires shifting of some criteria from the moderate treatment need to the great treatment need category (Fig. 5).