Neurotoxicity is a rare but well recognized unwanted effect of lithium therapy (Moore, 1972; von Hartitzsch et al, 1972; Tyrer and Shopsin, 1980) which tends to be associated with high serum lithium levels. In clinical practice it is normally assumed that once signs of toxicity are detected and treatment stopped, serum levels will return to normal. We report here a patient who developed toxic symptoms while her serum levels were in the therapeutic range but who developed severe neurotoxic sequelae and high serum levels after treatment was stopped.