Abnormal amino acid metabolism is sometimes observed among patients with diabetes mellitus. Of many amino acids, alanine and branched-chain amino acids such as valine, leucine, isoleucine show characteristic changes. In diabetic ketoacidosis, plasma concentration of alanine decreases and that of branched-amino acid increases and the oxidation of branched-amino acids is enhanced. Splanchnic amino acid uptake is generally higher in diabetics and this level is partially restored by exercise. Some glycosylated proteins are used to estimate the condition of diabetes mellitus. Increment of urinary glycosylated amino acid excretion is reported in diabetics. Plasma homocysteine, reactive vascular-injuring amino acid, increases in diabetics with nephropathy. Those abnormal amino acid metabolism would be restored after good glycemic control is obtained.