PURPOSE: The oral health-related quality of life (OHRQoL) of preschool children with Early Childhood Caries (ECC) was used to assess the outcome of dental treatment. STUDY DESIGN: Children (3.56±1.31years) affected by ECC were selected. Anamnesis, clinical and radiographic exams were performed. Data about sociodemographic aspects, OHRQoL and dmf-t index were collected. The Brazilian version of Early Childhood Oral health Impact Scale (B-ECOHIS) was applied in two time intervals: before and after 30 days of treatment. B-ECOHIS scores and effect size (ES) were used to evaluate the OHRQoL of preschool children. The Student's-t test was used for comparison between the B-ECOHIS averages, considering the following aspects: gender, age, socioeconomic status, severity of caries and type of treatment. RESULTS: The mean dmf-t was 6.25±4.20 and no differences between boys (6.00±4.32) and girls (6.83±4.35) (p=0,942) were found. Pain and psychological problems among children and parents feeling upset and guilty were the most frequently reported impacts at baseline. There was a greater impact on girls (17.67±8.68) than on boys (13.30±10.53) (p<0.001); and on children younger than 4 years (16.71±9.96) (p<0.05). The highest B-ECOHIS scores were observed in treatments involving dental extractions and space maintainers. There was no difference between the total B-ECOHIS scores of subjects from middle (16.24±10.30) and lower socioeconomic classes (15.97±10.26). The total scores of B-ECOHIS and its domains decreased after 30-day follow-up. Dental rehabilitation of preschool children showed a large effect size for these subjects (ES=1.19) and their families (ES=1.00). CONCLUSION: Dental treatment resulted in significant improvement of the preschool children's OHRQoL.