Rat NADP-dependent leukotriene B-4 12-hydroxydehydrogenase (Ltb4dh) catalyzes olefin reductions for some activated alkenes at the expense of NADPH in the absence of a flavin cofactor. Unlike flavoprotein alkene reductases, where net trans-addition of hydrogen has been consistently observed, Ltb4dh reduced both enantiomers of perillaldehyde to the same cis-product. To uncover the reason for this unexpected result, the stereochemical courses of perillaldehyde reductions by Ltb4dh were determined by deuterium labeling followed by H-2 NMR analysis. These data showed unequivocally that Ltb4dh mediated net trans-addition of hydrogen to (R)-perillaldehyde but followed the opposite stereochemical course (net syn-addition) for (S)-perillaldehyde. To the best of our knowledge, such divergent stereochemical pathways for a single enzyme have not previously been reported.