Lady Gaga is being sued by a top songwriter who claims he played a big part in her becoming a huge star - but says he's not been paid enough for it.
Rob Fusari says he co-wrote some of Lady Gaga's most famous songs and helped her with her name and look.
He also claims he helped her get her first record deal.
Lady Gaga hasn't commented on the case yet. But in the past she's said she came up with her name when she was 19 and she "stuck out like a sore thumb".
Lady Gaga, who's 23, said in an interview in 2009: "I was Gaga from the time that I was 19 through my first record deal."
"I always dressed like that before people knew me as Lady Gaga. I was always that way."
Mr Fusari, who's also a music producer, is claiming that Lady Gaga dropped him as soon as she got famous.
Lady Gaga's real name is Stefani Germanotta
He thinks he's owed £20m because of all the work he says he did for her.
He has been paid around £403,000 by Lady Gaga and her people, but he says he's owed a lot more.
Lady Gaga is one of the most successful singers around at the moment. Her debut album The Fame has been number one in the UK and America.