In case of dehydration, lithium can cause acute intoxication. This picture is mainly manifested by neurological disorders that can go as far as coma, digestive disorders, hydroelectrolytic disorders, and cardiovascular disorders. We report the case of a patient followed for bipolar disorder for 20 years and treated with lithium for 14 years and who presented an acute lithium intoxication resulting from a diabetes insipidus. Our objective is to underline the importance of good hydration and strict monitoring of lithium levels especially in situations favouring dehydration, notably the polyuria of diabetes insipidus.