An 80-year-old male visited the hospital as an outpatient with a head injury sustained in a traffic accident. Brain computed tomography incidentally revealed a left frontal lobe tumor measuring 5 cm in a diameter. The patient had a history of taking chlormadinone acetate (a progesterone agonist) prescribed several years previously as treatment for benign prostatic hypertrophy. The tumor was seen as an isointense lesion on T(1)-weighted magnetic resonance (MR) images with enhancement by gadolinium, and as a heterogeneously hyperintense mass on T(2)-weighted MR images. The tentative diagnosis was left frontal meningioma attached to the sphenoid ridge or sphenoid plane. The patient was managed conservatively because of his advanced age and no symptoms or progression were observed during a 9-month follow-up period. The medication for benign prostatic hypertrophy was changed from chlormadinone acetate to naftopidil (an alpha-2-blocker) about 9 months after his first presentation. The patient presented again 2 years later complaining of dizziness. Computed tomography and MR imaging performed at this time revealed remarkable regression of the tumor. The signal intensity change with regression of the tumor on T(2)-weighted images was observed as a hypointense lesion. Thus, we wish to emphasize that treatment of meningiomas, especially those diagnosed incidentally, must be based on a thorough consideration of any history of hormonal therapy with prostate disease.