OBJECTIVE To determine retrospectively the long-term radiographic outcome of root canal treatments and root canal retreatments with unintentional root canal overfilling. METHODS A total of 220 root canal treatments (143 primary/77 retreatments) with postoperative unintentional canal overfilling and performed by two endodontists during their 45 years of private practice were included in the study. Lateral condensation techniques and nine different sealers were used. Overfilling was confirmed with a postoperative periapical radiograph and patients were scheduled regularly for recall visits. Average recall time was 4.86 years (maximum = 30 years). Two calibrated observers evaluated the radiographs and determined the long-term outcome using the PAI score pooled in a 3-category scale. The persistence or resorption of the extruded material was registered. The Kappa coefficient (K) was calculated and a logistic regression was used for further analysis. Odds ratios and their 95% CI were estimated. RESULTS The level of inter-observer agreement was 66.1%. Primary root canal treatments had a significantly (P = 0.015) greater rate of success (91.6%) than retreatments (81.8%). Tooth location (P = 0.019) was the only other factor that significantly affected the outcome. The type of extruded material, and its resorption or persistence did not relate to the outcome. Persistence of extruded material was significantly affected by tooth location and recall time after treatment. CONCLUSIONS The outcome of root canal treatment with unintentional canal overfilling was not associated with the type of extruded material or its resorption or persistence. The persistence of extruded material did not relate to a favourable or unfavourable outcome.