Medication errors are an important cause of patient morbidity and mortality, of which there have been few reports in psychiatry, especially in the UK. Our aim was to examine the nature, frequency and potential severity of prescribing errors in UK mental health units in a prospective, 1 week survey of errors detected by pharmacy staff in nine NHS trusts. Pharmacists checked 22036 prescription items. In total, 523 errors meeting the study definition were detected (2.4% of prescription items checked). Prescription writing errors (77.4%) were most common, while decision-making errors accounted for 22.6% of errors. In 280 (53.5%) cases the prescribed drug had been administered before the error was detected. Most errors were of doubtful or minor importance but 22 (4.3%) were deemed likely to result in serious adverse effects or death. The error detection rate varied fourfold between trusts. Prescribing errors are fairly common in psychiatry. A small proportion of errors have the potential for serious harm. Pharmacy staff have an important role to play in their management.