Mark discovers that he has an 18-year-old daughter, Sloan Riley (Leven Rambin), who arrives seeking his support after becoming pregnant and being evicted by her mother. Mark allows her to move in with him, which upsets Lexie. When Lexie makes it clear that she knew that Mark would choose his own daughter over her, she puts an end to their relationship. Sloan experiences difficulties in her pregnancy, so Mark takes her to LA to be treated. He and Addison once again sleep together, but when he comes back he tells Lexie because he doesn't want to lie to her and wants them to get back together, but when she tells him she slept with Alex it puts an end to their relationship. When Sloan decides to give her baby up for adoption, Mark and Callie offer to raise it together. Sloan unwillingly agrees, but then quietly leaves Seattle, leaving Mark distraught. He enters into a relationship with cardiothoracic surgeon Teddy Altman (Kim Raver), despite the fact she is in love with their trauma colleague, Owen Hunt (Kevin McKidd). Sloan returns unexpectedly and gives birth to a son in Mark’s apartment. She reconsiders the adoption, but when Mark reassures her that he will support her no matter what, she gives the baby to an adoptive couple from Washington.