Introduction: People with intellectual disabilities and co-occurring psychopathology have a relatively high likelihood to engage in aggressive behavior. Nevertheless, structured clinical assessment of aggressive behavior, including when and where it occurs, is scarce in this population. Methods: On three wards specializing in the care for people with mild to borderline intellectual disability and co-occurring psychopathology, staff members completed the Staff Observation Aggression Scale-Revised adapted for people with intellectual disabilities (SOAS-R-ID) during a nine-month period, every time they witnessed aggressive behavior. Results: Based on 236 SOAS-R-ID forms, it was found that aggressive incidents were most common on Thursdays, and on two specific moments of the day (between 9-11 a.m. and 7-9 p.m.). The aggressive behavior was often exclusively of a verbal nature and was usually targeted against staff members (77.1% of the incidents). The Interclass (Pearson) Correlation Coefficient agreement between observers on the total score of the SOAS-R-ID was 0.72. Correlation between the judgment of the severity of aggressive behavior made by the staff members and the SOAS-R-ID severity scores was moderate (r = 0.40), but significant. Conclusions: Because aggressive behavior appeared to result often from interactions between the client and staff member or other clients, these interactions might be an important starting point for interventions. Structured clinical assessment of aggressive behavior can help to devise and test the effects of interventions. The SOAS-R-ID seems to be a clinically useful instrument and could therefore help to reduce the frequency of these incidents in the future.