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 profoundly shape professionals' understandings of social and health problems, as well as of their own roles and responsibilities and of what interventions and outcomes are possible. The data consist of 48 tape-recorded weekly team meetings among professionals in a supported housing unit targeted for clients with both mental health and substance abuse problems. It was found that professionals reason about the relationship between these two problems in four different ways: (1) substance abuse causes or makes mental health problems worse; (2) substance abuse eases mental health problems; (3) mental health problems cause or make substance abuse worse; or (4) 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.