Annual health checks are recommended for patients with severe mental illness (SMI) as they are at high risk of cardiovascular disease. Ideally, these health checks should be carried out in primary care. Practice nurses are already competent in carrying out physical health checks, but might have misconceptions about mental illness, which is a barrier to offering the service. We used a mirror imaging study to establish the effectiveness of a training package for practice nurses that aims to address common misconceptions about the physical health of people with SMI. This 2-hour training package (Northampton Physical Health and Wellbeing Project) was delivered to eight practice nurses. Their misconceptions and beliefs were assessed before and after training. Motivation to work with community mental health workers was assessed after training. The practice nurses involved in the study rejected commonly held misconceptions about the physical health of people with SMI after training. Their attitudes towards their role in providing health checks appeared to be modified in a positive direction. Their motivation to work with community mental health workers also seemed to be enhanced. The Northampton Physical Health and Wellbeing Project training was effective in modifying practice nurses' misconceptions about physical health in people with SMI.