The purpose of this study was to evaluate nurses' reporting of medication errors.Improvement in medication error reporting is one of the major challenges in today's health care environments.This was a descriptive survey of nurses working in medical, surgery, orthopaedic, gynaecology and obstetric wards in hospitals affiliated to Iran University of Medical Sciences, using stratified multistage sampling. Data were collected between November 2008 and May 2009 using a researcher-designed questionnaire. Data were described and explored using Kruskal-Wallis, one-way analysis of variance and Mann-Whitney's test.The response rate was 93% (n = 286).The mean number of medication errors 'reported' per nurse during 3 months was 1.33 [standard deviation (SD) = 4.18, median = 0, interquartile range (IQR) = 0-0, range = 70-0]. The mean number of errors made was 19.5 (SD = 30.27, median = 10, IQR = 1-25, range = 181-0). None of the individual and organisational characteristics reported were significantly related to reporting of medication errors. Failure to record vital signs (e.g. pulse, blood pressure etc.) before and after administering certain medicines was the most frequently reported medication error.More errors were made than were reported and this requires further investigation.The reporting of medication errors is a problem in health care systems worldwide, including Iran. Considering the significant difference between the nurses' actual and reported medication errors, managers should monitor medication errors. Educational initiatives are needed to improve understanding of the importance of medication administration.