OBJECTIVE To assess orthodontic treatment need prior to treatment and 5 years postretention in a large university sample. METHODS A random sample of 920 patients (400 male and 520 females) from the Department of Orthodontics and Oral Biology, University of Nijmegen, the Netherlands, were included in this study. Dental casts were evaluated using the Index of Orthodontic Treatment Need (IOTN) at the pretreatment (TP) and 5-year postretention stages (T5). The mean age at TP was 12.2+/-3.0 years and at T5 21.6+/-3.1 years. At TP and T5 the aesthetic component (AC) and the dental health component (DHC) were assessed. The difference between TP and T5 was compared for males and females and tested by the t-test. The changes in AC, DHC and treatment need categories were described at TP and at T5. RESULTS On the basis of combined AC and DHC treatment need categories, 83% of the patients fell into the "Definite need" category prior to treatment and 10% of the treated patients were categorised as having "Definite need" 5 years postretention. No significant differences were found between males and females for the change in AC and DHC between TP and T5. CONCLUSIONS The results indicate that the policy used in the department for patient selection gives priority to patients with a high treatment need. The results also provide a general impression of treatment outcome utilising the IOTN to analyse the change in the treatment need categories.