This study investigated the long-term clinical outcome of root canal treatment. 240 root-treated teeth (n = 61 patients) were initially classified on the basis of radiographic presence/absence of initial apical periodontitis (IAP) and clinical data. The final outcome measure was the periapical healing (healed/disease). The outcome at 6-9 months was correlated with the outcome at 10 years following treatment. Prognostic factors for the periapical healing were assessed. Extraction data were recorded. Univariate and multivariate logistic regression analysis was used to identify risk indicators for apical periodontitis (AP) development. Chi-square analysis was performed to evaluate a possible relationship between the 6-9 months outcome and the final outcome related to IAP. Mean observation time was 14 ± 3.7 years. Survival rate was 84.6% and healing rate was 79% (10-19 years). Predictors of outcome (p < .05) were considered statistically significant. Multivariate logistic regression analysis showed that initial pulpal and periapical status and the quality of root canal filling as assessed two-dimensionally were independent predictors of outcome. The 6-9 months evaluation appears to be an indicator for the final outcome of primary root canal treatment both in the presence and in the absence of IAP. An initial radiolucency associated with an unsatisfactory quality and extent of root canal filling significantly diminishes the possibility of achieving long-term radiographic success. For those with uncertain healing at 6-9 months (91%), clinicians should consider the high healing rate when estimating the prognosis and adjust the decision making accordingly.