In chronic renal failure both HbA1 and HbA1c levels have been reported to be elevated. In order to investigate the causes of such increase we measured HbA1 (cation-exchange chromatography), blood urea nitrogen, arterial blood pH, plasma bicarbonate, phosphatemia, serum iron and serum ferritin before dialysis in 60 uremic patients receiving long term hemodialysis. The increased levels of HbA1 do not correlate with glucose intolerance, phosphatemia, blood urea nitrogen, time averaged concentration of urea, serum iron and serum ferritin. On the contrary the presence of a highly significant correlation between HbA1 and arterial blood pH (p less than 0.001) and between HbA1 and plasma bicarbonate (p less than 0.001) seems to emphasize a major role for acidosis in increasing the HbA1 levels in uremic patients on long term hemodialysis.