The Cubs had been in the market for a left-handed reliever to firm up their bullpen, and Grabow has been on their radar for a while. Sean Marshall was the only lefty in the bullpen.
Before Thursday's game, Cubs general manager Jim Hendry told reporters talks had been heating up in the last day as the non-waiver trade deadline expires Friday afternoon.
Grabow has been a big name on the trade market as teams try to stock up for the stretch drive. The lefty set-up man is 3-0 with a 3.42 ERA for last-place Pittsburgh, with 41 strikeouts and 28 walks in 47 1/3 innings. He has two blown saves and 16 holds. The 30-year-old has a career 4.04 ERA, and a 3.29 ERA in 42 appearances against the Cubs.
ESPNChicago.com writer Bruce Levine contributed to this story.