This paper highlights the benefits, issues and difficulties involved in the development of schizophrenia case registers on general practitioner clinical information systems. The development of the registers included identifying those patients with an active diagnosis of schizophrenia by cross matching primary and secondary information systems. Patients identified with an uncertain diagnosis and/or not known to secondary mental health services were assessed by a community psychiatric nurse; both the assessment process and the outcome of the assessments are presented in the paper. The project also included a clinical audit of antipsychotic medication prescribing, which indicated the need for improvements in prescribing and recording practice between primary and secondary care. The paper concludes with a discussion on the advantages and disadvantages of case registers in primary care from the perspective of the experience gained in this project together with key learning points relating to the construction of severe mental illness (SMI) registers in primary care.