BACKGROUND In this retrospective cohort study of the effectiveness of interceptive orthodontic treatment, we compared patients receiving interceptive orthodontic treatment with untreated control subjects. METHODS Models were scored by using the index of complexity, outcome and need (ICON). Control models (n = 113) were archival and were selected based on malocclusion in the early mixed dentition and no orthodontic treatment during the subsequent 2 years. The patients (n = 133) were in the mixed dentition and consecutively treated in the University of Bergen orthodontic clinic. Initial ages were 9.4 years (+ or - 1.4) for the treated group and 9.3 years (+ or - 0.8) for the control group. The treatment took a mean of 27.2 months (+ or - 16.3) for the patients; the control group was observed for a mean of 24.4 months (+ or - 3.6). Subject Groups were matched for age, need, complexity, duration, and all ICON components except spacing (P <0.006) and crossbite (P <0.000). RESULTS ICON scores decreased after treatment by 38.8% (P <0.0001) from 54.9 (+ or - 16.6) to 33.6 (+ or - 16.1). The controls were unchanged, with ICON scores of 54.0 (+ or - 14.8) and 54.2 (+ or - 16.9). Improvement grades were different (P <0.0001), with most controls categorized as "not improved or worse" (89.4%), whereas only 36.1% of the treated group were in that category. However, there were increases in the "minimal," "moderate," and "substantial" improvement categories for the treated subjects (22.6%, 21.1%, and 17.3%, respectively). The controls did not change in any ICON component and worsened in crowding (P <0.007), whereas the patients improved in esthetics, crowding, crossbite, and overbite (P <0.007). CONCLUSIONS These results indicate that interceptive orthodontic treatment is effective for improving malocclusion but does not produce finished-quality results.