BACKGROUND: Medication errors can arise both during prescription and administration (dispensing and distribution) of drugs. Little is known about types of medication errors in Norwegian hospitals. MATERIAL AND METHOD: All medication errors reported at St. Olav's Hospital from 1 July 2002 to 30 June 2006 were reviewed and analysed. RESULTS: 610 reports were identified. The most common cause of reporting (39 %) was prescription of a different dose from the one prescribed. Other frequent causes were administration of a different drug than the one prescribed (17 %), inadvertent subcutaneous infusion of an intravenous drug (15 %), and that the drug was given to another patient (12 %). The errors were almost exclusively reported by nurses. In 107 cases (18 %), precautions had been taken to reduce the extent of injury after the error had been identified. The causes of errors could be classified in three main categories: Nonvigilance caused by stress, lack of appropriate routines or violation of them, and lack of appropriate skills/negligence. INTERPRETATION: Changes of routines, improved education in existing routines, and increased pharmacological competence may contribute to prevention of medication errors.