OBJECTIVE: To evaluate A1C for screening and diagnosis of undiagnosed type 2 diabetes defined by oral glucose tolerance testing in clinical and general populations. RESEARCH DESIGN AND METHODS: A1C cut offs (< or =5.5% to rule out diabetes; > or =7.0% to rule in diabetes) were derived from a clinical group (Melbourne Pathology [MP] group: n = 2,494; undiagnosed diabetes 34.6%) and then evaluated in a population-based sample (AusDiab group: n = 6,015; undiagnosed diabetes 4.6%). RESULTS: For diabetes in the MP and AusDiab groups, A1C at 5.5% gave sensitivities of 98.7 and 83.5%, while A1C at 7.0% gave specificities of 98.2 and 100%, respectively. Many (61.9-69.3%) with impaired A1C (5.6-6.9%) in both populations had abnormal glucose status. CONCLUSIONS: A1C < or =5.5% and > or =7.0% predicts absence or presence of type 2 diabetes, respectively, while at A1C 6.5-6.9% diabetes is highly probable in clinical and population settings. A high proportion of people with impaired A1C have abnormal glucose status requiring follow-up.