The level of glycosylated albumin has been determined in the serum of normal and diabetic subjects after purification of the albumin to apparent homogeneity. The sugar was released from the albumin preparations as 5-hydroxymethylfurfural (HMF) after 24 h of hydrolysis in 2 N acetic acid at 92°C, and it was assayed by the thiobarbituric acid reaction. The mean value for glycosylated albumin, expressed as picomoles of HMF per nanomole of albumin, obtained from 10 normal control and 65 diabetic subjects, was 64 and 124, respectively. The level of glycosylated albumin correlates with the mean blood glucose concentration (N = 55, r = 0.715), but not with the fasting blood sugar concentrations. Moreover, a linear relationship was observed between the amounts of glycosylated hemoglobin (HbAla−c) and glycosyl-albumin (n = 74, r = 0.88). In an insulin-treated diabetic patient, there was a different temporal relationship between blood glucose concentrations and glycosylated hemoglobin and albumin levels. While HbAla−c was lowered by only 15% after 20 days, glycosylated albumin had dropped by more than 50% during the same time. Our results indicate that glycosylated albumin might provide a valuable tool to assess the average blood sugar levels between shorter intervals, since the turnover of serum albumin is considerably faster than that of HbAla−c.