Type 1 diabetes (T1D) is an autoimmune disease characterised by immunity to pancreatic beta-cell autoantigens, associated with beta-cell destruction leading to insulin deficiency and hyperglycaemia. The rigorous definition of an autoimmune disease requires evidence that an autoantigen elicits pathological immune responses. Using criteria for the pathogenicity of an autoantigen, we examine the evidence for proinsulin as an autoantigen in T1D. We conclude that proinsulin satisfies these criteria. As a corollary, proinsulin is a potential immunotherapeutic tool for the prevention of T1D.