Hemifacial spasm (HFS) due to intracranial mass lesions is rare. Most cases are thought to be due to compression of the facial nerve by small vessels near the root of the facial nerve. A survey was undertaken of all botulinum toxin investigators to determine the incidence of imaged mass lesions causing HFS. Responders contributed information on 1676 patients with HFS. Of this group, nine tumors were reported for an incidence of 0.54% of patients. However, of this group only 52.5% underwent computed tomography (CT) or magnetic resonance (MR) scanning so the incidence of tumor causing HFS could be as high as 1.0%. No one tumor type was predominant, and most patients were women older than 50 years of age. The incidence compares with another large series of HFS patients in which one tumor was found in 367 patients. The authors also report as an illustrative case a 26-year-old man with HFS due to a presumed lipoma of the cerebellopontine angle. This diagnosis can be made with increased certainty with MR scanning. If the incidence of unsuspected diagnostically significant mass lesions is 1 in 200 patients with HFS referred for botulinum toxin injection, the cost of detecting one such lesion would be $100,000 at an average imaging cost of $500 per MR imaging or CT examination. Although mass lesions are uncommon, any patient with HFS whose general clinical course could justify intervention should be considered for imaging studies to rule out treatable conditions other than vascular compression.