The term 'off-label' prescribing refers to the use of a drug outside the terms of its Marketing Authorization, including prescribing for an unlicensed indication. The aims of the study were to determine the frequency of off-label prescriptions for mood stabilizers (lithium and antiepileptics) among inpatients of a large psychiatric hospital, the nature of the off-label clinical indications in use and whether patients had been informed about the off-label usage.A cross-sectional survey of inpatients aged 18-65 years at St Andrew's Hospital, Northampton, UK and interviews with consultant psychiatrists about off-label usage of mood stabilizers were carried out. Of the 249 patients studied, 75 (30.1%) were receiving one or more mood stabilizers, of which 71 (94.7%) were off-label. The most frequently cited off-label indications for mood stabilizers were: prophylaxis of mood swings (48 cases), treatment of aggression (31), manic symptoms (10), antipsychotic augmentation in treatment-resistant schizophrenia (7) and post-traumatic stress disorder (6). Lithium was prescribed infrequently. The reasons for this are discussed. Although in most instances the psychiatrist was aware the drug was being used off-label, in less than one-third of cases had the patient been informed of this, partly because of anticipated difficulties in their understanding the off-label concept, but also because of concerns that this information could adversely affect compliance. The off-label prescription of mood stabilizers is very common in psychiatry and such usage benefits patients. When prescribing off-label, psychiatrists should consider the evidence that the drug is likely to be effective for the unlicensed indication. Where there is limited evidence of benefit, a trial of the drug, with clinical monitoring, may be indicated. Patients should be fully informed about their medication, and this includes information that the prescription is off-label. Pharmacists can assist this process. The off-label concept may be difficult for some patients to understand.