Drug prescription errors are a common cause of adverse incidents and may be largely preventable. The incidence of prescription errors in UK critical care units is unknown. The aim of this study was to collect data about prescription errors and so calculate the incidence and variation of errors nationally. Twenty-four critical care units took part in the study for a 4-week period. The total numbers of new and re-written prescriptions were recorded daily. Errors were classified according to the nature of the error. Over the 4-week period, 21,589 new prescriptions (or 15.3 new prescriptions per patient) were written. Eighty-five per cent (18,448 prescriptions) were error free, but 3141 (15%) prescriptions had one or more errors (2.2 erroneous prescriptions per patient, or 145.5 erroneous prescriptions per 1000 new prescriptions). The five most common incorrect prescriptions were for potassium chloride (10.2% errors), heparin (5.3%), magnesium sulphate (5.2%), paracetamol (3.2%) and propofol (3.1%). Most of the errors were minor or would have had no adverse effects but 618 (19.6%) errors were considered significant, serious or potentially life threatening. Four categories (not writing the order according to the British National Formulary recommendations, an ambiguous medication order, non-standard nomenclature and writing illegibly) accounted for 47.9% of all errors. Although prescription rates (and error rates) in critical care appear higher than elsewhere in hospital, the number of potentially serious errors is similar to other areas of high-risk practice.