Sir: We write to add our wholehearted support to Ann Barker for raising various concerns about dosage information contained in the British National Formulary (BNF) and how they have come to be used. (Psychiatric Bulletin, 1993, 17, 557). For some time now, in relation to the use of lithium medication we have been airing similar concerns about BNF and MIMS (Monthly Index of Medical Specialities). Both publications set out their aims and scope in their preface, for rapid reference and for use as a prescribing guide. Nonetheless, the publications seem to acquire a legal standing as, for example, "BNF maximum". The contents should be accurate and up to date so as to enjoy professional confidence and cred ibility. Otherwise how else could one reconcile major differences between BNF and MIMS as in the maximum suggested dose of injection Depixol where BNF suggests a maximum of 400 rng weekly and MIMS "up to 300 mg every 2 weeks". We are pleased that a Royal College Consensus Group is looking into these issues. The group may wish to examine the accuracy and current validity of relevant entries in BNF and MIMS so that medical practitioners are better informed and their patients better served let alone legal implications. Regarding lithium medication, the problem is with dose, side effects, toxicity and contra-indications, for which the information is outdated and sometimes conflicting. The Third British Lithium Congress held in Wolverhampton in September 1992 established a working group to produce a consensus report on proposed guidelines for good clinical practice to deal with the problems out of date information in the BNF and MIMS. (Lithium Prophylaxis: Proposed Guidelines for Good Clinical Practice.