Miliary dissemination is common in tuberculosis, but is an extremely rare form of brain metastasis. It is mainly found in patients with primary lung cancer (small cell and adenocarcinoma). Here, we presented a case of miliary metastases of lung adenocarcinoma to the brain without lesion enhancement on MRI after administration of contrast. A 38-year-old Chinese male was diagnosed with lung adenocarcinoma and received chemotherapy monthly for 6 months. At one month after completion of chemotherapy, the patient presented with headache, dizziness, and vomiting. Brain MRI revealed numerous, disseminated, tiny, rounded cystic high-signal intensity lesions on T2-weighted images, and low-signal intensity lesions on T1-weighted images, with no enhancement. In addition, a high signal on T2-weighted images and uneven enhancement with contrast in the hypophysis were noted. A right frontal lobe biopsy revealed miliary metastases originating from primary lung adenocarcinoma, which was consistent with the pathological finding of a bronchial biopsy. However, the patient and his family requested supportive treatments only, and he died 3 months after the diagnosis. In summary, this case indicates that when imaging findings are not consistent with the most likely cause of miliary brain metastasis, a biopsy is necessary to make a definitive diagnosis.