The title compound, 1-hydroxy-1-phosphono-3-(1-pyrrolidinio)propylidene-1-phosphonate, C₇H₁₇NO₇P₂, is a member of the bisphosphonate class of drugs. As a zwitterion, it possesses a negative charge on one of the PO₃ groups and a positive charge on the pyrrolidine N atom. A zwitterion makes a contact with a neighbouring ion through the hydroxyl O atom and two phosphonyl O atoms, one each from two different PO₃ groups. Hydrogen bonding involves O–H⋯O and N–H⋯O interactions; the former are involved in the formation of head-to-head dimers, while the latter join the dimers into a chain running along the crystallographic b axis.