Hyperthyroidism is associated with increased psychiatric morbidity. It may alter the clinical course of alcohol withdrawal syndrome. We report a 69 year old man who presented prolonged alcohol withdrawal syndrome associated with hyperthyroidism. Initially, he developed typical alcohol withdrawal syndrome including tremor, disorientation, delirium and visual hallucination of small animals. Thyroid function tests revealed a free triiodothyronine (T3) of 6.1 pg/dl (range, 3.0 to 5.8), a free thyroxine (T4) of 2.3 ng/dl (range, 0.85 to 2.15) and a thyroid stimulating hormone (TSH) of 0.003 microU/ml (range, 0.3 to 4.0), and thiamazole was administered. Even after a month, he continuously presented persecutory delusion, auditory hallucination and cognitive dysfunction. Although these symptoms did not respond to the medication including antipsychotics, they totally passed away after the thyroid function reached down to the normal level (free T3 3.0 pg/ml, free T4 1.1 ng/dl, TSH 0.004 microU/ml). In addition, cognitive function was recovered to the normal level as he scored 28/30 on the Mini Mental State Examination. We propose that hyperthyroidism contributed to the occurrence of psychotic symptoms and cognitive dysfunction.