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Lithium is one of the first choice agents for long-term prophylaxis and treatment of acute episodes of bipolar disorders. It has a narrow therapeutic index; the target plasma concentration ranges from 0.6 to 1.5 mEq/L. Lithium toxicity can occur at levels> 1.5 mEq/L. There are three types of lithium poisoning: acute, acute or chronic and chronic. Factors that increase the risk for chronic toxicity in previously stable patients include other medications, illness, volume depletion. We report chronic lithium toxicity associated with dehydration in an elderly patient. Our patient was a 63-year-old man, with his first manic episode experienced5 years ago and used lithium carbonate 900 mg/day and quetiapine fumarate 50 mg/day for five years. His premorbid nature was hyperthymic. He was admitted to our psychiatric outpatient clinic presenting with slurred speech, confusion, drowsiness, coarse tremor, hyperreşexia, apathy, ataxia, and bradycardia. Lithium plasma level was elevated to 2.69 mEq/L. The patient and his wife denied any taking lithium overdose however, 2 weeks ago, water intake and appetite decreased due to stress factors. Mental status examination was euthymic. Laboratory tests for blood serum glucose, serum calcium, sodium, and potassium, were well within normal limits. His blood urine and creatinine levels were slightly increased, not the levels as in renal failure. He had sinus bradycardia (HR: 50). His diffusion MR imaging was normal. He had no comorbid diseases or concurrent medications that can increase the risk of developing lithium intoxication. Because the patient had seriously high lithium levels and associated clinical conditions, he was consulted to nephrology for hemodialysis. With the first hemodialysis, his lithium level decreased to 2.1 mEq/L, than with second hemodialysis to 0.9 mEq/L, and his neurological manifestations recovered by hemodialysis. It is now generally accepted that elderly patients do not tolerate lithium as well as younger patients and can develop serious adverse effects more rapidly while taking lower doses or at lower serum levels. Dehydration, which can be ignored in clinical practice, is important metabolic conditions that increase the risk of lithium toxicity. The best approach is certainly the use of preventive measures. Only increased physician awareness and the early use of effective treatment, namely dialysis, will prevent the mortality and protracted morbidity associated with this condition. This case illustrates some of factors that lead to lithium intoxication, advanced age, some metabolic disease, renal diseases, additional medication and dehydration. The most important factor is dehydration in elderly patients for lithium intoxication.