A patient is described who simultaneously suffered from hallucinations and one-and-a-half syndrome following surgical resection of a cavernous angioma located on the fourth ventricle floor. A 30-year-old male was admitted having a 10 year history of progressive weakness and hypesthesia of the right extremities. Neurological examination revealed right hemiparesis and hemihypesthesia. A computerized tomography (CT) scan showed a well circumscribed mixed-density area at the left portion of the pontine tegmentum. Magnetic resonance imaging (MRI) indicated a mixed intensity core surrounded by hypointense regions on T2-weighted images. A red mulberry-like tumor was found, during surgical procedures, on the mid-and left-portion of the fourth ventricle floor. The tumor was totally resected and the histological diagnosis was cavernous angioma. Visual hallucinations occurred several times following surgery, being subsequently described as images of moving worms, a dump truck next to the bed, and a bed falling from the ceiling. The patient soon understood these objects were not real. He simultaneously exhibited left lateral gaze palsy associated with left internuclear ophthalmoplegia i.e., one-and-a-half syndrome. Three months after surgery the visual hallucinations disappeared but the one-and-a-half syndrome was incompletely resolved. According to the neurological signs and the MRI results, the lesion site was considered to extend from the left lower pons to the midbrain. Previously reported operations of cavernous angioma of the fourth ventricle floor are also described.