Lithium remains to be the gold standard in the treatment of mood disorders. This study presents a case treated with lithium for an extended period with a good response. Following an increase in creatinine levels, further investigation of renal dysfunction revealed polycystic kidney disease. Lithium was used prior to the diagnosis of polycystic kidney disease, resulting in the unique opportunity to examine the effects of lithium on kidneys with polycystic kidney disease. Within this context, this study also discusses the pharmacokinetics of lithium, and its possible relation to cyst formation in polycystic kidney disease.