A ruling by the Court of Justice of the European Union in MB v Secretary of State for Work and Pensions Case C-451/16 [2018] All ER (D) 135 (Jun) that EU law precludes UK legislation which requires a woman who had previously been a married man to be no longer married to a woman in order to be able to claim a state retirement pension as from the statutory pensionable age applicable to women is examined. This case has significant implications on how countries recognise changes to gender.