In mental health care, clinicians’ treatment decisions are expected to be based on the formulation (i.e., exploration of the causing and maintaining mechanisms) of the client’s problems. Previous research showed two things: clinicians’ case formulations mainly contain descriptive information instead of explanatory information and it is unclear to what extent treatment decisions are actually based on case formulations. In this study, we tested whether the complexity of client problems influences case formulation quality and we investigated to what extent case formulations explain treatment decisions for simple and complex client problems. Results show that case formulations for complex problems were of lower quality than those for simple problems. Also, we found that case formulations are only weakly associated with treatment decisions, for both types of problems. We conclude that clinicians give higher quality case formulations when these may be least needed, that is: for simple cases, for which an empirically supported treatment is available. Clinicians appear to base treatment decisions on descriptions of overt client symptoms rather than on case formulations.