Grether possesses one of the most valuable collections of 20th-century art in the world. Her collection reportedly includes more than 600 pieces and includes works by Pablo Picasso, Paul Cézanne, Salvador Dalí, Francis Bacon and Alberto Giacometti. The collection is kept in a converted printing factory which is also her home. Grether owns Bacon's Triptych, May–June 1973 (one of Bacon's three "Black Triptychs") which she purchased at auction in 1989 for $6.3 million, a record price for a Bacon painting at that time. She is also believed to own three other Bacon triptychs from the 1970s.