Routine use of Health of the Nation Outcome Scales (HoNOS) has not produced the anticipated benefits for people using mental health services. Four HoNOS-specific reasons for this are: low relevance to clinical decision-making; not reflecting service user priorities; being staff-rated; and having a focus on deficits. More generally, the imposition of a centrally chosen measure on the mental health system leads to a clash of cultures, since front-line workers do not need a standardised measure to treat individuals. A better approach might be to use research from the emerging academic discipline of implementation science to inform the routine use of a standardised measure that is chosen by the people who will use it and hence is more concordant with existing clinical processes. This is illustrated using a case study of successful implementation of the Camberwell Assessment of Need (CAN) in community mental health services across Ontario, Canada.