The 26-year-old 'Bad Romance' singer opened the milliner and close friend's spring/summer 2013 collection on the catwalk in a sheer pink cape over a nude catsuit with a pair of platform boots.
GaGa then took to the front row next to Treacy wearing a floral headpiece before changing into a pair of mouse ears embellished with Swarovski crystals.
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Other guests at the fashion show included Nick Cave, Kelly Brook, Vivienne Westwood, Kim Cattrall, Bryan Ferry, Dita Von Teese and Big Pink's Robbie Furze.