There has been controversy over whether to radiologically follow up or use local treatment for asymptomatic small-sized brain metastases from primary lung cancer. For brain tumors without local treatment, we evaluated potential factors related to the brain progression and whether systemic therapy controlled the tumor. We analyzed 96 patients with asymptomatic small-sized metastatic brain tumors from lung cancer. These underwent a radiologic follow-up every 2 or 3 months without local treatment of brain metastases. The pathologies of the tumors were adenocarcinoma (n= 74), squamous cell carcinoma (n= 11), and small cell carcinoma (n= 11). The primary lung cancer was treated with cytotoxic chemotherapy (n= 57) and targeted therapy (n= 39). Patients who received targeted therapy were divided into first generation (n= 23) and second or third generation (n= 16). The progression-free survival (PFS) of brain metastases and the overall survival (OS) of patients were analyzed depending on the age, tumor pathology, number, and location of brain metastases, the extent of other organ metastases, and chemotherapy regimens. The median PFS of brain metastases was 7.4 months (range, 1.1–48.3). Targeted therapy showed statistically significant PFS improvement compared to cytotoxic chemotherapy (p= 0.020). Especially, on univariate and multivariate analyses, the PFS in the second or third generation targeted therapy was more significantly improved compared to cytotoxic chemotherapy (hazard ratio 0.229; 95% confidence interval, 0.082–0.640;p= 0.005). The median OS of patients was 13.7 months (range, 2.0–65.0). Univariate and multivariate analyses revealed that the OS of patients was related to other organ metastases except for the brain (p= 0.010 and 0.020, respectively). Three out of 52 patients with brain recurrence showed leptomeningeal dissemination, while the recurrence patterns of brain metastases were mostly local and/or distant metastases (94.2%). Of the 52 patients who relapsed, 25 patients received local brain treatment. There was brain-related mortality in two patients (2.0%). The intracranial anti-tumor effect was superior to cytotoxic chemotherapy in the treatment of asymptomatic small-sized brain metastases with targeted therapy. Consequently, it becomes possible to determine the optimal timing for local brain treatment while conducting radiological follow-up for these tumors, which do not appear to increase brain-related mortality. Furthermore, this approach has the potential to reduce the number of cases requiring brain local treatment.