The term 'peduncular hallucinosis' has become part of the neurologist's lexicon, despite relatively little clinicopathologic verification of the syndrome. First described in 1922 by L'Hermitte, the syndrome involves vivid hallucinations and signs of lesions in the midbrain. It should be emphasized that 'peduncular' was translated directly from 'pedonculaire', which refers not only to the cerebral peduncles, but to the entire midbrain. The midbrain, or mesencephalon, includes the superior and inferior colliculi, the tegmentum, the cerebral peduncles, the substantia nigra, and other structures. However, a case of hallucinosis has now been seen in which magnetic resonance imaging showed a lesion apparently limited to the right paramedian thalamus. The patient, a diabetic, began to see a variety of animals in his house, and at one time saw ''military people'' marching in front of him. During examination the patient asked that ''those girls'' leave the room, as he was not properly dressed. Although hallucinating, the patient was alert and cooperative. There were no hallucinations on the third hospital day, and the patient was sent home. Upon examination a week later, the patient was free from hallucinations. Peduncular hallucinosis has traditionally been attributed to lesions in the mesencephalon. However, it is very uncommon for lesions to occur in this area without also affecting the paramedian thalamus. The present patient, whose lesion did not extend into the midbrain or the cerebral peduncles, is an important indication that the role of the thalamus in hallucinosis may be more significant than has been previously thought. (Consumer Summary produced by Reliance Medical Information, Inc.)