Lesnar (5-2) hasn't fought since losing the title to Cain Velasquez at UFC 121 in October 2010. He'll welcome the newcomer Overeem (35-11), who has held belts in multiple organizations but has never fought in the UFC.
"It's very good to finally be in the UFC," said Overeem, who weighed in at 263 pounds. "As for tomorrow, it will be short. I've predicted a first, and if not in the first, a second-round knockout."
Diaz (14-7) came in one pound overweight on his first attempt, weighing 157 pounds. He was given one hour to cut to 156, which he did successfully. Cerrone (17-3) came in at 156.
Welterweight contender Jon Fitch (23-3-1) looks to rebound from a majority draw decision against B.J. Penn in February. The tie snapped a five-fight win streak for Fitch, who hadn't lost since coming up short in a title fight against Georges St. Pierre in August 2008.
Fitch came in right at the 171-pound limit. His opponent, former Oklahoma State University wrestler Johny Hendricks (11-1) weighed 170 pounds.
Brett Okamoto covers mixed martial arts for ESPN.com. Follow him on Twitter.