People with severe mental illness are at increased risk of heart disease and diabetes. First episode psychosis (FEP) patients tend to engage in health-risk behaviours, typically consuming an unhealthy diet and engaging in low levels of physical activity. Protection Motivation Theory proposes that risk perceptions are the most proximal determinants of such behaviour. Understanding the risk perceptions of FEP patients may therefore have the potential to inform health behaviour change interventions. This thesis reports findings from a prospective cohort study, in which FEP patients completed a battery of assessments (including a theory-derived risk perception questionnaire) at baseline, and health behaviour was measured twelve months later. Three preliminary studies were undertaken to identify a parsimonious health risk questionnaire. The Health Risk Appraisal Study (Study 1) modelled the relationship between risk perceptions, diet and physical activity. Risk perceptions remained stable between baseline and follow-up, despite increases in BMI, which suggest a declining health state. Although risk perceptions did not predict health behaviour at either time point, findings demonstrated that patients underestimated their health risks. The Cognitive Deficits Study (Study 2) showed that cognitive deficits account for a small but significant proportion of the variance in risk perceptions, though risk perceptions did not mediate relationships between baseline cognitive deficits and 12-month health behaviour. The Insight Study (Study 3) showed that insight into mental health status was associated with greater perceived health risks. The Interview Study (Study 4) used qualitative methods to explore reflections on health-related experiences. While patients valued their physical and mental health and wellbeing, and had some knowledge of their health risks, during the acute phase of illness, physical health was a lesser concern than mental wellbeing, housing, employment and education. Risk perception models may have limited utility for understanding health among FEP patients. Physical health promotion campaigns must recognise and contextualise health concerns within the broader goals and priorities of FEP patients.