The stereoselective synthesis of either trans- or cis-3,5-disubstituted pyrazolidines is accomplished via Pd-catalyzed carboamination reactions of unsaturated hydrazine derivatives. The products are obtained in good yield with up to > 20:1 diastereoselectivity. Stereocontrol is achieved by modulating the degree of allylic strain in the transition state for syn-aminopalladation through a simple modification of the substrate N-2-substituent. The pyrazolidine products can be further transformed to 3,5-disubstituted pyrazolines via deprotection/oxidation, or to substituted 1,3-diamines via N-N bond cleavage.