A 42-year-old male presented with the complaint of mild left facial numbness. Magnetic resonance imaging demonstrated a solid tumor in the interpeduncular cistern and a huge arachnoid cyst in the left middle cranial fossa. The tumor appeared isointense to the surrounding cerebral gray matter on T1-weighted images and hyperintense to that on T2-weighted images. The tumor was partially resected. Histological findings were characteristic of hamartoma. The mild left facial numbness was probably due to compression of the left trigeminal nerve by the arachnoid cyst. Asymptomatic hypothalamic hamartomas may occur in adults with atypical clinical presentations.