Background: Existing data on adding internal mammary nodal irradiation (IMNI) to the regional nodal fields are inconsistent. Methods: Randomized trials investigating the addition of IMNI to standard adjuvant radiation were identified. Hazard ratios (HRs) and 95% confidence intervals (CI) were extracted for overall-survival (OS), breast cancer specific-survival (BCSS), and disease-free survival (DFS) as well as distant-metastasis free survival (DMFS). The odds ratios (ORs) for regional and loco-regional recurrence, non-breast cancer mortality, secondary non-breast cancer, contralateral breast cancer, and cardiovascular morbidity and mortality were also extracted. Results: Analysis included five trials comprising 10,994 patients, predominantly with higher risk, lymph node positive disease. Compared to the control group, IMNI was associated with significant improvement in OS (HR = 0.91, p = 0.004), BCSS (HR = 0.84, pp= 0.01), and DMFS (HR = 0.89, p = 0.02). IMNI was also associated with reduced odds for regional (OR = 0.58, pp = 0.04). The odds for cardiotoxicity were not statistically significantly higher (OR = 1.23, p = 0.07). There were comparable odds for cardiovascular mortality (OR = 1.00, p = 1.00), non-breast cancer mortality (OR = 1.05, p = 0.74), secondary cancer (OR = 0.95, p = 0.51), and contra-lateral breast cancer (OR = 1.07, 95% 0.77–1.51, p = 0.68). Conclusions: Compared to the control group, the addition of IMNI in high-risk patients is associated with a statistically significant improvement in survival, albeit with a magnitude of questionable clinical meaningfulness. View Full-Text