Background: Inpatient aggression in treatment facilities for persons with intellectual disability (ID) can have aversive consequences, for co-clients and staff, but also for the aggressors themselves. To manage and eventually prevent inpatient aggressive incidents, more knowledge about their types and characteristics is necessary. Method: In four facilities, totalling 150 beds, specialized in the treatment of adults with mild ID or severe challenging behaviour, aggressive incidents were registered during 20 weeks using the Staff Observation Aggression Scale-Revised. Characteristics of auto-aggressive and outwardly directed incidents and differences in their incidence in male and female clients in these facilities were compared. Results: During the observation period of 20 weeks, 639 aggressive incidents were documented. Most of these (71%) were outwardly directed, predominantly towards staff, while most of the remaining incidents were of an auto-aggressive nature. Of the 185 clients present during the observation period, 44% were involved in outwardly directed incidents (range per client 1-34), and 12% in auto-aggressive incidents (range per client 1-92). Auto-aggressive and outwardly directed incidents differed regarding source of provocation, means used during the incident, consequences of the incident and measures taken to stop the incident. The proportion of men and women involved in each type of incident was comparable, as well as the majority of the characteristics of outwardly directed incidents caused by men and women. Conclusions: Although approximately half of all clients were involved in aggressive incidents, a small minority of clients were responsible for the majority of incidents. Therefore, better management and prevention of aggressive incidents for only a small group of clients could result in a considerable overall reduction of aggressive incidents in treatment facilities. Comparability of aggressive behaviour in these facilities shown by men and women and differences in characteristics of auto-aggressive and outwardly directed incidents are discussed.