Healthcare givers, and their managers, have a duty to monitor the frequency and types of accidents and untoward incidents which occur to patients in their care. The purpose of this study was to systematically examine accidents and incidents (AIs) involving patients in a mental health service unit. Data were collected on all inpatient and day patient AIs over a 6-month period (January to June 1991). The data were then analysed to identify: (a) the nature of AIs occurring in the unit; (b) the frequencies of the AIs, and (c) the times at which AIs occurred. In the wards for elderly patients, an increase of AIs was noted from 05.00 hours; after this time the frequency of AIs remained fairly constant until 20.00 hours when a decrease occurred. In the other wards, a rise in AIs could be seen from 07.00 hours and a fall occurred after 01.00 hours. However, within the time period of 07.00-01.00 hours, further patterns were noted in the data for all AIs in the wards for non-elderly patients, as well as for incidents involving aggression and self-harm. Possible reasons for these findings are suggested, and recommendations made for the examination of some nursing practices with the unit.