Peduncular hallucinosis (PH) consists of formed and coloured visual images, which the patient knows are unreal; it is often associated with lesions of the pons, midbrain and diencephalon. A 72-year-old man had noted the sudden onset of visual hallucinations one year before, specifying the time and body position in a 4-week, 24-h diary. Thereafter, he underwent video-polysomnography (VPSG), brain magnetic resonance imaging (MRI), angiography (MRA), proton spectroscopy (1H MRS), and single photon emission tomography (SPECT). Patient’s diaries and VPSG showed a strong clustering of hallucinatory experiences during the evening/night time while lying in supine position, similar to hypnagogic hallucination and sleep paralysis in supine position. Repeated episodes of REM sleep behaviour disorder (RBD) occurred during the night. MRI and MRA showed an elongated and dilated left internal carotid artery displacing the left subthalamus upwards, and 1H MRS relatively decreased N-acetyl-aspartate in the left subthalamus. Brain SPECT during PH revealed hypoperfusion in the right temporal region and hyperperfusion in the left occipital and right opercular regions (the latter possibly related to the patient’s awareness of unreality). PH resolved with serotonergic (citalopram) therapy.