Background: A key component of cognitive therapy for psychosis is the development of a meaningful and shared formulation of the onset of the psychosis. Therapists bring an understanding of the development of psychotic symptoms based on theoretical models and try to marry these with the person's own experience and explanations. However, an important question is whether this understanding is compatible with the explanations held by the client. Aims: This study investigated what factors people believed led to the onset of their psychosis. Method: A Q set of potential causes for psychosis was identified from a literature search and interviews with people with differing experiences of psychosis. From this, 58 potential causes of psychosis were identified. Twenty-one people who had experienced a psychotic breakdown then ranked these explanations as possible causes. Results: Using Principle Components Analysis four main factors were identified as perceived causal factors for the onset of psychosis. These factors were described as: a) drug usage; b) traumatic experiences in adulthood; c) personal sensitivity; and d) developmental vulnerabilities. Conclusions: This study revealed that people with psychosis have different explanatory frameworks for the onset of their difficulties. This work is helpful as this effort to understand the person's own understanding of their problems is a first stepping stone towards a collaboratively generated formulation that may be helpful in building a therapeutic alliance, engagement in treatment, and ultimately in improved outcome. It is likely that these different explanatory frameworks could lend themselves to different treatment approaches.