Things get ugly between Lisa Vanderpump and Lisa Rinna during the upcoming three-part Real Housewives of Beverly Hills season six reunion special.

During the episode, Rinna takes aim at Vanderpump, claiming she encouraged her to bring up Munchausen Sydrome.

“We’re at my birthday and you came … I was thrilled to see you,” Rinna tells Foster in a sneak peek shared by Us Weekly on Thursday, citing her birthday dinner at Il Cielo. “You came with no makeup. You left. [Vanderpump’s husband] Ken had said you looked terrible and [my husband] Harry said, ‘You know what? That’s not OK. This woman is sick and we need to stand by her.’”

“Ken didn’t say she looked terrible to her face … he was observing that she didn’t look good,” Vanderpump notes.

“Harry said, ‘We need to support this woman whether she’s sick or not.’ … And you go, ‘There goes our ‘f--king’ story line,’” Rinna alleged.

At that point, Foster breaks down in tears as Vanderpump attempts to deny Rinna's allegations.

“I’m not done!” Rinna continues. “I got a phone call from you and you encouraged me to say ‘Munchausen.’ I’m getting this out! You called me at home … I’ve got the phone records … 10 f--king phone calls! I’m sorry, you said you’ve never called me … [but] you encouraged me to say this.”

During the season, it was Rinna who brought up the idea of Munchausen, claiming the issue had been brought to her attention by a friend in regard to Foster's Lyme disease diagnosis. However, according to Rinna, it was Vanderpump who wanted the lable out there, not her. 

The three-part Real Housewives of Beverly Hills reunion will begin airing on Bravo on Tuesday, April 19, at 9 p.m.