The study aims to present a case of atypical poisoning with lithium carbonate in a 57-year-old woman treated for bipolar affective disorder with lithium carbonate for about 30 years. The patient was admitted to the hospital with significant agitation. An important finding obtained from the family interview was the patient's significant weight loss over the past year. In the hospital, the patient received haloperidol and clonazepam. Laboratory tests showed a very high blood lithium concentration of 3.79 mmol/l [N: 0.6─1.2 mmol/l] and elevated serum concentrations of creatinine (3.6 mg/dl) and urea (110 mg/dl). The patient was transferred to the toxicology department, where hemodialysis was performed and intensive treatment initiated. Despite the rapid decrease in lithium levels, her condition gradually deteriorated. The patient died on the fifth day of hospitalization. The autopsy revealed polycystic kidney disease (PKD). During the preparation of the medico-legal report on the correctness of the medical treatment, it was assumed that the cause of death was lithium carbonate poisoning in the course of advanced chronic kidney disease due to PKD, probably a consequence of long-term lithium therapy. The analysis of medical records revealed that despite her psychiatrist's recommendation, the patient had been refusing the monitoring of lithium levels for the past 18 years. This case demonstrates that both psychiatrists and toxicologists should be aware of possible lithium poisoning upon the deterioration of renal function. Therefore, assessment of renal function should be an integral part of monitoring lithium therapy.