A 58-year-old woman presented with 4 lesions of pulmonary adenocarcinoma which were curatively resected. No adjuvant therapy was needed because the lesions were all in the early stage. Magnetic resonance (MR) imaging of the brain, taken 10 months after the curative surgery, disclosed focal enhancement along the right central sulcus. Cerebrospinal fluid examination was normal. The lesion remained stable for 16 months, then began to spread out. She underwent open biopsy and the histological diagnosis was leptomeningeal metastasis from adenocarcinoma. She was treated with gefitinib at first because the lesion expressed epidermal growth factor receptor domain mutation. However, the lesion continued to enlarge, so she underwent whole cranial irradiation which was not effective. She died 2 years 10 months after the first detection of the metastasis. Early histological diagnosis is important even though the MR imaging lesion is not likely to be metastasis after curative surgery of the primary lesions.