The characteristics of two samples of people with learning disabilities and severely challenging behaviour were examined. The experimental sample comprised subjects who had been referred to two specialist challenging behaviour services in separate counties. The comparison sample comprised subjects from the same counties who were not referred or accepted for specialist intervention. No significant differences were found between the samples in terms of age, gender, social impairment, dependency characteristics, ability levels and mental health. Significant differences between them were found in the nature and severity of aberrant behaviour presented, as assessed by their carers. The implications of these findings are discussed with regard to how the term ‘severely challenging behaviour’ is defined in practice.