Wilson's disease (WD) shows a wide heterogeneity in symptoms. In this case report we present hypersomnia as a symptom of WD. The male patient's complaints as fatigue, decreased level of concentration, and highly increased demand of sleeping started at his age of 21 years. No abnormality was found at physical examination. A moderate elevation in liver function tests was found, but all the other laboratory findings were within the normal range. The marked hypersomnia was verified by 24-h cassette EEG polisomnographic monitoring. No abnormality was found at physical examination. EEG, brain CT and MRI were normal. Neither toxic nor infectious disease was detectable. The diagnosis of WD was based on decreased coeruloplasmin level, increased baseline and forced urinary excretion of copper, and decreased level of serum copper. Kayser-Fleischer ring was not detectable. D-penicillamine (DPA) was introduced. At 8-10 months after the initiation of the therapy the patient's complaints gradually resolved. The control sleep record 14 months after the initiation of the DPA therapy was normal. Five years later the patient is currently on penicillamine treatment and he is free of any symptom.