In vivo, glucose bound to protein by aldimine linkage undergoes an Amadori rearrangement to form ketoamine (generically termed fructosamine). The present study was undertaken to investigate the clinical usefulness of fructosamine determinations in diabetic control. The fructosamine assay (in accordance with the methd of Johnson et al.) was based on the ability of the ketoamine-linked glucose of glycated protein to reduce the dye nitroblue tetroazolium and measurement of the subsequent change in optical density. The serum fructosamine concentration was measured in 52 diabetic and 16 control subjects. The diabetic sera showed significantly higher levels of fructosamine than the control sera (1.79±1.3 (mean±2 SD) mmol/l vs. 1.04 ± 0.4 mmol/l; p<0.001). Seruml fructosamine concentrations were significantly correlated with glycated albumin (r=0.708, p<0.001), glycated haemoglobin (HbA1c)(r=0.751, p<0.001) and fasting plasma glucose levels for the preceding two weeks (r=0.474, p<0.01) and four weeks (r=0.737, p<0.001). These data and a case stndy suggested that serum fructosamine concentration was an index of intermediate term (two to four weeks) blood glucose control. This fructosamine assay is simple and cheap, and may provide an additional useful tool for monitoring blood glucose control.