L-Ornithine lactam is an endogenous metabolite formed by the dehydration of the non-proteogenic amino acid L-ornithine. Excessive levels of L-ornithine lactam in plasma is diagnostic for a rare autosomal recessive disorder of the urea cycle. Rather whimsically, L-ornithine lactam is used in cosmetics as a skin whitening agent and moisturiser. Ornithine lactam is a biosynthetic precursor of sinefungin. Little is published on the biological activity of ornithine lactam.