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Kleine-Levin Syndrome (KLS) is a rare disease characterized by recurrent episodes of hypersomnia associated with cognitive and behavioral disturbances, compulsive eating behavior and hypersexuality. Each episode lasts for one or two weeks, and affected people are entirely asymptomatic between episodes. We aimed to report a 13 year-old girl with a possible Kleine Levin Syndrome. She was admitted to emergency clinic, for her complaints of acute onset disorganized and inappropriate behavior and meaningless talking. Her mother acknowledged that the symptoms occur dafter tonsillitis. She experienced increased sleepiness (sleeping around 20 hours/day) and appetite, she was easily upset, she had excessive talking and rapid shifting of ideas and she began to have motor hyperactivity. In emergency room, her physical examination was normal, the laboratory findings, including complete blood count, hepatic and renal functions, lumber puncture, electroencephalography and cranial magnetic resonance imaging did not reveal any abnormality. During the psychiatric interview, she had a pressured and disorganized speech, she was anxious and uncooperative, her affect was tearful, she was crying intermittently. Her place and time orientation was poor. She had şight of ideas and was extremely fearful of dying and being separated from her parents. She did not have any known medical illness. Her previous psychiatric history was negative for traumatic or stressful life events or substance abuse. Her developmental milestones were in normal timeline. Her peer relationships were good; she had fairy good academic achievement. Her family history was positive for bipolar disorder; her two maternal uncles and grandmother had lithium responsive-bipolar disorder. The initial treatment included a combination of quetiapine (gradually increased up to 600 mg/day) and lorazepam (1 mg/day for 5 days). She responded considerably well to the initial treatment within a week; however, her symptoms tended to have an episodic manner. Her mental status examination was completely normal and she was functioning very well, without any psychiatric symptoms, between the episodes. Within 3-4 weeks, somehow overlapping with the last days of her menstrual cycle, her symptoms reappeared. The increment of quetiapine to 1200 mg/day led to a prominent decrease in the symptom intensity, however, the episodes persisted. By the 5th episode lithium was introduced. The treatment protocol including lithium (with a serum level of around 0.95 mEq/lt) and quetiapine 200 mg/day resulted in more indistinct episodes, gradually. For the last 4 months, she has been free of any episodes. At the first glance, due to the clinical picture and the positive family history, the diagnosis of bipolar disorder seemed to be likely. When the symptoms of hyperphagia, hypersomnia, and the intense disorganized behavior were taken into account, we considered KLS as another possible diagnosis. KLS is a clinical diagnosis with no available definite laboratory test. In patients with relapsing-remitting episodes of severe hypersomnia, cognitive impairment, apathy, derealization, and psychiatric and behavioral disturbances, it should be considered as a possible diagnosis. Due to its episodic nature and clinical response to mood stabilizers, it should also be kept in mind as a criteria for differential diagnosis in adolescents with suspected bipolar disorder.