Professionals in human service work are at the centre of complicated client cases. The ways client cases are constructed and the problems explained form the basis for professionals' assessments, decisions, actions and interventions. In this article the ways professionals make sense of dual diagnosis client cases are examined. Applying the concept of causal accounting it is argued that 'theories of cause' are embedded in professional discourse and that they profoundly shape professionals' understanding of the social and health problems, their roles and responsibilities and possible interventions and outcomes.The data consist of 48 tape-recorded weekly team meetings among the professionals in a supported housing unit targeted for clients with both mental health and substance abuse problems. Four different ways to reason for the relations between these two problems are found: (i) substance abuse causes or makes mental health problems worse; (ii) substance abuse eases mental health problems; (iii) mental health problems cause or make substance abuse worse; and (iv) good mental health reduces substance abuse. Causal account research makes visible the ways professionals do institutional work by categorizing clients, accounting for responsibilities as well as assessing their work and clients' achievements according to moral expectations of a 'good' professional and a worthy client.