Recent reviews of the effects of lithium on renal function have generally concluded that no clinically significant effect on glomerular function is apparent even after many years. Yet, a handful of studies over the last 3 years indicate that a small percentage of lithium-treated patients may develop rising levels of creatinine in serum after a decade or more of treatment. In the current sample of 82 bipolar patients treated in an affective disorders clinic, 3 (3.7%) were found to have developed creatinine levels in serum greater than 2.0 mg/100 ml from baseline levels that were within normal limits. One of these patients has progressed to chronic renal failure and hemodialysis, making him the second probable reported case of lithium-induced chronic renal failure. No common risk factor for renal disease among these patients was apparent. As increasing numbers of patients are treated with lithium for a decade or more, previous conclusions as to the benign effects of long-term lithium treatment on renal function may need to be revised. Regular monitoring of creatinine levels in serum and medical consultation if the level rises and remains above 1.6 mg/100 ml are recommended.