Inter Milan are leading AC Milan 1-0 in the Derby della Madonnina at half-time.
The Nerazzurri made the breakthrough as early as the fourth minute through Walter Samuel.
Milan goalkeeper Christian Abbiati misjudged the flight of an Esteban Cambiasso free kick and Samuel scored with a diving header.
The Inter centre-back's Argentine compatriot Diego Milito almost doubled his side's advantage three minutes later.
The 33-year-old striker intercepted Abbiati's pass out of his penalty area and shot at the Rossoneri goal, but it was saved by the goalkeeper.
Inter continued to dominate proceedings at San Siro, with Milito proving to be a handful for the Milan defence.
However, Milan came close on 13 minutes as Riccardo Montolivo flashed a shot just wide of Samir Handanovic's goal.
Massimiliano Allegri's side had another chance on the half-hour mark, with Kevin-Prince Boateng latching on to a cross from Daniele Bonera only to shoot wide.
Three minutes later the visitors went close, Milito placing a shot-cum-cross along Milan's six-yard box but there was no-one in an Inter shirt to turn it home.
Montolivo thought that he had given Milan the lead six minutes from the interval with a brilliant volley, but it was ruled out because of a foul from Urby Emanuelson on Handanovic.
Bojan Krkic then tested Handanovic with a shot from inside the area on the stroke of half-time.
