OBJECTIVE: We evaluated the role of a single measurement of HbA(1c) in a diabetes case finding in hospitalized patients with random hyperglycemia at admission. RESEARCH DESIGN AND METHODS: From 20 March to 31 July 2000, 508 patients admitted through the emergency department of one hospital were tested for random hyperglycemia (plasma glucose [PG] >125 mg/dl). Consenting patients with hyperglycemia (without preexisting diabetes or on corticosteroids) underwent testing for HbA(1c) levels, two fasting PG levels, and an outpatient oral glucose tolerance test (OGTT) if necessary. RESULTS: Of the patients, 50 (9.8%) met the inclusion criteria. Of these, 70% (n = 35) completed the study, and 60% (n = 21) were diagnosed with diabetes. Patients with diabetes had higher HbA(1c) levels than subjects without diabetes (6.8 +/- 0.4 vs. 5.3 +/- 0.1%, P = 0.002). An HbA(1c) level >6.0% was 100% specific (14/14) and 57% sensitive (12/21) for the diagnosis of diabetes. When a lower cutoff value of HbA(1c) at 5.2% was used, specificity was 50% (10/21) and sensitivity was 100% (7/14). CONCLUSIONS: In acutely ill patients with random hyperglycemia at hospital admission, an HbA(1c) >6.0% reliably diagnoses diabetes, and an HbA(1c) level <5.2% reliably excludes it (paralleling the operating characteristics of the standard fasting glucose measurements); however, the rapidity of the HbA(1c) level can be useful for diabetes case finding and treatment initiation early in the hospital course.