Leptomeningeal carcinomatosis (LMC) is a relatively rare complication of advanced-stage cancer. The diagnosis of LMC is based on clinical and radiologic findings and cytologic examinations. The diagnosis may be difficult because the clinical findings may be subtle initially, and the specificity of the diagnostic modalities is low. A 41-year-old male patient with metastatic lung adenocarcinoma presented with headache, confusion, and treatment-resistant seizures. There was no intraparenchymal mass lesion or leptomeningeal contrast enhancement in brain magnetic resonance imaging. The presence of LMC was demonstrated in a cytopathologic examination of cerebrospinal fluid. The seizures were controlled with antiepileptic treatment, but the patient was referred to the palliative care center due to poor oncologic prognosis.