A synthesis of l-proline where all of the ring methylenes are stereoselectively labelled with deuterium is described. A catalytic deuteration of protected 3,4-dehydro- l-proline using transition metal catalyst followed by RuO 4-oxidation gave a [3,4-D 2]pyroglutamic acid derivative. A syn-selective deuteration of the aminal derived from the pyroglutamate with Et 3SiD–BF 3·OEt 2 furnished (2 S,3 S,4 R,5 S)-[3,4,5-D 3]proline. The present procedure is also applied to the synthesis of the corresponding (2 S,3 S,4 R,5 R)-isomer.