To determine the objective orthodontic treatment need of a group of school children and a referred population using the Index of Orthodontic Treatment Need (IOTN) and establish the relationship between subjective and objective orthodontic treatment need.The study groups were 250 school children and 157 children referred to the orthodontic unit of the Obafemi Awolowo University Teaching Hospital Complex (OAUTHC). They assessed their need for treatment, their dental casts were also assessed by the examiner (K.A.K) using the IOTN.The professional assessment of treatment need of the children in the school population based on the Aesthetic Component of IOTN were 62.8% no need, 30% moderate need and 7.2% great need for orthodontic treatment, the referred population had 19.7%, 36.3% and 43.9% respectively. The Dental health component resulted in 66% no need, 20% moderate need and 14% great need for treatment in the school population. These percentages were 20.4%, 16.6% and 63% respectively in the referred population. Statistically significant differences were found between subjective and professional assessment of orthodontic treatment need in both populations. There were no significant gender differences in objective orthodontic treatment need.The results of the study indicate that there is a difference of opinion on orthodontic treatment need between laypersons and professionals. The IOTN reliably records deviant occlusal traits can be used to prioritize treatment need.