Lithium is widely used in psychiatry, mainly in the prophylaxis of manic-depressive illness. It is usually prescribed long term and because of its side effects, low therapeutic index and the possibility of toxicity, it is necessary to monitor serum lithium levels, thyroid function and renal status. Clinical guidelines are unclear; Vestergaard et al (1982) recommended that serum lithium and creatinine be checked every two to four months and thyroid function every six months. However, Crammer et al (1982) suggest that once stable levels are established, serum lithium need be checked only in response to clinical indications. The most recent edition of a widely read psychiatric text states that levels should be checked each six to eight weeks for as long as the patient continues on lithium (Loudon, 1988). Accepted standard clinical practice is thus unclear. It should be emphasised that these guidelines are not based on research evidence.