Trans-4-hydroxy-L-proline is an optically active form of 4-hydroxyproline having L-trans-configuration. It has a role as a mouse metabolite, a plant metabolite and a human metabolite. It is a tautomer of a trans-4-hydroxy-L-proline zwitterion. L-Hydroxyproline, one of the hydroxyproline (Hyp) isomers, is a useful chiral building block in the production of many pharmaceuticals.