The purpose of this study was to explore a pattern of underreporting within a psychiatric general hospital in Japan. All the medication errors reported online in 2010 were analysed. This research was approved by the university and the study hospital. There were 651 incidents related to medication errors. Medication error rate per 1000 patient days was 2.14 (range: 0.45-6.05). Medication error rates between two acute care wards with comparable case and staff mix differed. A low proportion of intercepted near-misses and low medication error rates around mealtime in acute care 1 were suggestive of under-reporting. Two dementia care wards with low medication error rates had no report of intercepted errors, which was also suggestive of underreporting. Ward-specific medication error rates and patterns are useful to identify wards with potential underreporting of medication error within the hospital.