The main goal of this study was to investigate whether staff members vary in their frequency reports on challenging behaviour concerning the same client. Because staff's approaches of challenging behaviour are affected by a range of staff characteristics, we hypothesised that these staff characteristics could explain this variability between staff members. We used questionnaires to investigate the influence of staff's age, gender, experience in working with people with intellectual disabilities, working hours, professional qualifications, sort of education, and their emotional reactions and beliefs regarding the challenging behaviour. This study involved 152 staff members and 51 clients with severe or profound intellectual disabilities who displayed self-injurious behaviour, stereotyped behaviour, and/or aggressive/destructive behaviour. A part of the variability between staff's reports on the frequency of challenging behaviour was indeed explained by differences between the staff members. Working hours, internal attribution, gender, and experience in working with people with severe or profound intellectual disabilities turned out to be influencing variables. Summarising, staff members differ in their reports on the frequency of challenging behaviour. To get an accurate picture of a client's challenging behaviour, perceptions of several staff members are needed. (Contains 4 tables and 1 figure.)