The clinical significance of measuring glycated fibrinogen (G Fbg) in plasma was studied using a simple, rapid method for determining plasma G Fbg developed by the authors. This method is based on modified fibrin clot formation for purification of plasma Fbg, and G Fbg was estimated by improved colorimetric fructosamine assay. The values of plasma G Fbg were expressed as glycated polylysine equivalents (mumol/l). In this clinical study, 106 examinees who underwent a screening test for diabetes mellitus (DM) because of glycosuria or hyperglycemia were investigated. According to the WHO diagnostic criteria, they were divided into 3 groups: non DM group (n = 35), impaired glucose tolerance (IGT) group (n = 31) and DM (non-treated) group (n = 40). In addition, healthy controls (n = 40) and treated diabetic patients (DM treated) were involved. Plasma G Fbg/Fbg, fasting plasma glucose (FPG), fructosamine and glycated hemoglobin (HbA1c) were also analyzed in subjects of the 5 groups. Non-treated DM patients showed high levels of plasma G Fbg, which were significantly higher than those of any other groups. G Fbg levels in IGT group were significantly higher than those in healthy controls or the non-DM group. Together with a significant positive correlation between G Fbg and FPG, these results indicate that the levels of plasma G Fbg depend on plasma glucose levels.(ABSTRACT TRUNCATED AT 250 WORDS)