The Briton was followed home each time by the American, Colin Edwards, on a Castrol Honda with the home rider Pier-Francesco Chili third in both races.
Fogarty, on his Ducati, began the day by winning a spectacular opening race, holding off Edwards and Suzuki's Chili in a thrilling 18-lap affair at the high-speed track.
The Blackburn rider, who has put himself in a good position to land a fourth world title, pipped his Texan opponent Edwards, who had begun the race on pole position, by just 0.12 sec at the finish to stretch his championship lead to 48 points before the day's second race.
Behind Edwards, winner of both races at Monza last season, was Chili, who earned Suzuki a place on the podium. The Australian Troy Corser, Fogarty's team-mate and the 1996 champion, was a disappointing fourth after clocking the fastest lap, but remained second in the standings on 144 points.
In the second race Fogarty and Edwards crossed the finish line side by side, but Fogarty was declared the victor by an even narrower margin this time - 0.005 sec.
WORLD SUPERBIKE (Monza, Italy):
First race: 1 C Fogarty (GB) Ducati 32min 13.009sec, 2 C Edwards (US) Castrol Honda) 32:13.129, 3 P-F Chili (It) Suzuki 32:13.556, 4 T Corser (Aus) Ducati 32:24.140, 5 A Slight (NZ) Castrol Honda 32:26.962, 6 N Haga (Japan) Yamaha 32:27.946.
Second race: 1 Fogarty 32min 18.285sec, 2 Edwards 32:18.290, 3 Chili 32:18.829, 4 Corser 32:26.548, 5 A Yanagawa (Japan) Kawasaki 32:30.734, 6 Haga 32:30.763.
Championship leaders (after five of 13 rounds): 1 Fogarty 217 pts, 2 Edwards 162, 3 Corser 157, 4 Yanagawa 126, 5 Slight 115, 6 Haga 95, 7 Chili 85, 8 G Lavilla (Sp) 58.Reuse content