The complexity of the Maillard reaction arises partly from multiple fragmentation reactions of the sugar moiety, constituting branch points in the reaction progress and establishing many parallel reaction pathways. Reactive intermediates produced by these processes are often alpha-oxoaldehydes. The formation of alpha-oxoaldehydes enhances and redirects glycating activity in the Maillard reaction since alpha-oxoaldehydes are up to 20,000-fold more reactive than glucose in glycation processes and are predominantly arginine-directed glycating agents. alpha-Oxoaldehydes bypass a requirement for a fructosamine precursor in the formation of advanced glycation end products (AGEs) since alpha-oxoaldehydes react with proteins (also nucleotides and basic phospholipids) to form AGEs directly. The major AGE formed from alpha-oxoaldehydes is generally a hydroimidazolone with other products-although for glyoxal, N(omega)-carboxymethylarginine is a major product. alpha-Oxoaldehyde formation also occurs in the absence of an amine substrate, particularly during heat processing of sugar solutions and lipid peroxidation processes-in the latter case, the glycation adducts are advanced lipoxidation products (ALEs). Hydroimidazolones are quantitatively important AGEs in cellular and extracellular proteins in physiological systems. Hydroimidazolone free adducts are liberated by cellular proteolysis and digestion. They are released into blood plasma for urinary excretion. Modification of arginine residues by alpha-oxoaldehydes may be particularly damaging since arginine residues have high-frequency occurrence in ligand and substrate recognition sites in receptor and enzyme active sites. Along with fructosamine formation, alpha-oxoaldehyde intermediates of the Maillard reaction represent a major source of damage to the proteome and genome.