BACKGROUND This prospective study was designed to determine the prognosis of second molars with external root resorption (ERR-M2s) caused by embedded third molars (EM3s) following EM3 removal and explore related predictors. METHODS This study consecutively enrolled 58 participants who had asymptomatic second molars with apical external root resorption caused by EM3s. EM3s were extracted, and follow-up examinations were conducted at 1 week, 4 weeks, and 6 months. The primary outcome was prognosis of ERR-M2s after third molar extraction without any further intervention, as assessed via clinical and radiographic examinations. Potential predictors (sex, age, number of roots, EM3s position, type and degree of external root resorption) of prognosis were analyzed via univariable and multivariable analyses. RESULTS A total of 63 ERR-M2s from 58 patients (16 males and 42 females; 19-57 years of age) were evaluated. At the last follow-up, 56 teeth (89%) remained asymptomatic and normal response to heat and cold tests. According to univariable analyses, ERR-M2s were more likely to remain normal response to heat and cold tests in younger patients. In the multivariable analysis, older age (odds ratio, 1.118; 95% confidence interval, 1.026-1.219; P < .05) and the type of all roots affected (odds ratio, 0.073; 95% confidence interval, 0.007-0.754; P < .05) were significantly associated with poor prognosis. CONCLUSIONS Asymptomatic ERR-M2s have a high probability of remaining normal pulp without further intervention after EM3 extraction, especially in younger patients. No intervention except follow-up and observation is necessary.