Hemoglobins A1c and A1a+b were measured by an automated chromatographic method in 11 control subjects, 100 diabetics and 30 subjects with renal failure not induced by diabetes. Hb A1c was higher in diabetics than in controls (8.26 +/- 0.31 versus 5.24 +/- 0.28, p less than 0.01) and strongly correlated with blood glucose values for the preceding 4 months. In poorly controlled diabetics, submitted to an intensive therapeutic program, Hb A1c decreased rapidly. Hb A1c was slightly but significantly elevated in the uremic, non-diabetic patients who were not submitted to periodic hemodialysis: 6.42 +/- 0.32, p less than 0.05. It returned, in hemodialyzed patients, to a level not significantly different from the control value. Hb A1a+b was elevated in diabetic subjects (2.43 +/- 0.04 vs. 1.55 +/- 0.01 in controls, p less than 0.001). It was also higher in the uremic patients (2.71 +/- 0.14, p less than 0.001). No decrease occurred in the hemodialyzed patients (3.27 +/- 0.31). Glycosylated hemoglobin values, as routinely estimated and expressed under the name of "Hb A1", should be interpreted with caution in patients with renal failure. Methods discriminating Hb A1a+b have to be used.