Mark supports Derek when Meredith comes close to dying after drowning, and the two are able to rekindle their friendship. It is eventually revealed that he and Derek grew up together and that he, having lost his mother as a child and his father emotionally neglecting him, considered Derek's mother as his maternal figure. As he was an only child and Derek was the only son, they became close friends and he was Derek's best man at the latter's wedding to Addison. He also forms a friendship with Callie, and develops an attraction to cardiothoracic surgeon Erica Hahn (Brooke Smith). Erica does not reciprocate his feelings, and he is supportive of Callie when she and Erica have a brief relationship. When Addison shows up at Seattle Grace for a case, Mark attempts to sleep with Addison again but she refuses. The nurses later start a club against him called Nurses United Against Mark Sloan. After a string of unresolved sexual tension on his part, Mark then finds himself seduced by Lexie Grey (Chyler Leigh), despite Derek's request that he keep their relationship platonic even before their relationship had begun. They begin a sexual relationship, which Lexie halts until he is willing to publicly admit to being her partner. Mark eventually agrees and tells Derek, who attacks him. Animosity exists between them for several episodes, before they once more repair their friendship. Mark and Lexie's relationship becomes increasingly serious. He meets her father, Thatcher (Jeff Perry); and at the end of season five, asks her to move in with him. Though Lexie initially declines, he remains committed to their relationship, and she agrees early in season six.