Fructose-lysine, which is formed by binding glucose to lysine, is degraded on acid hydrolysis into furosine. Furosine was determined by high-performance liquid chromatography according to the method of Schleicher et al. Furosine values were significantly higher in diabetic patients than in healthy subjects, and significantly correlated with haemoglobin A1 (HbA1) values. These results suggest that furosine, like HbA1, may become an indicator of long-term blood glucose control in diabetic patients and be useful in investigating diabetic complications on the level of tissue.