For more than one hundred years (about 1845-1950), lithium salts were used to treat disorders belonging to 'the uric acid diathesis' ('gouty diseases'). It was introduced into modern psychiatry as an antimanic agent, but its current use is mostly as a prophylactic in bipolar and unipolar manic-depressive illness. In the present context, however, this psychiatric use may, in some instances, create special nonpsychiatric problems such as lithium poisoning, renal diabetes insipidus, and weight gain. Moreover, most lithium patients are outpatients, so that medical complaints caused by the treatment are in most cases presented to the general practitioner, the neurologist, the nephrologist, the cardiologist, etc. rather than to the psychiatrist. Lithium treatment is frequently accompanied by both short-term and long-term side effects and some of these, e.g. the thyroid-depressing effect and leukocytosis, may be medically exploited. Like manic-depressive illness, some medical disorders such as certain types of headache have a periodic course and appear to respond favorably to long-term lithium treatment. Cooperation between researchers in biological psychiatry and nephrologists has resulted in the suggestion that lithium may be used as an indicator of proximal sodium and water reabsorption, which has later led to the suggestion that the renal lithium clearance is an indicator of proximal renal tubular function.