(FORTUNE Small Business) – Phone rings in Frank Olivieri's office (a desk, a couch, a filing cabinet, and a door opening onto a concrete stoop in South Philadelphia), and Olivieri picks up. "Tell me where you are, and I'll tell you how to get to where I'm at." Olivieri waits. "Take Broad Street northbound. Come to the 1200 block south, which is Wharton Street. You'll see a Mobil gas station on your right and a mural of Frank Sinatra. Make a right. Come five blocks down to Ninth. And get the hell out of the car."

Welcome to ground zero in the Philly cheese-steak wars. For the better part of a half-century (24 hours a day, seven days a week), Pat's King of Steaks, in business since 1930, and Geno's Steaks, the challenger since 1967, have stared each other down across this barren patch of South Philly pavement like Apollo Creed and Rocky Balboa. It's the twin-shrine mecca of greasy meat, drawing visiting rock stars, college students with a severe case of the munchies, and politicos on the trail. (To Olivieri, Al Gore is just "Al" and John McCain is an "awesome guy.") You want diamonds, go to 47th Street in Manhattan. You want a honkin' drippin' cheese steak, right here.

Both restaurants stake a claim to inventing the beloved sandwich. Pat, the original king of steaks, was Olivieri's great-uncle, a former street vendor who, the story goes, tossed some sliced beef on the grill because he was tired of eating hot dogs and so invented the steak sandwich. Geno's owner, Joe Vento, claims that he was the first to add cheese atop that sandwich, thereby inventing the classic Philly cheese steak. (Pat's later one-upped by introducing Cheese Whiz, which has since become the topping of choice.)

Pat's is the dowdier-looking joint of the two--wrapped in aluminum siding, festooned with Pepsi signs, accessorized with a sheet-metal awning. Geno's is the same idea but a little brighter. The menus are nearly identical, and the prices might well be fixed ($5 for a steak, $5.50 with cheese). Order like a rookie on either side of the street, and the guy behind the window will make loud fun of you. And together they sell an awful, awful lot of meat--about $10 million worth if you accept Olivieri's hard estimate ($5.5 million) and Vento's boast ("He's trying to catch up with me now").

While both Vento, 64, and Olivieri, 38, eat steaks often (though not as often as they used to; Vento's cholesterol topped off at 252 before he discovered chelation therapy), neither will touch the other's product, which is one reason they've never shared a meal. The other reason: They hate each other. Vento uses words like "arrogant" and "idiot" to describe his opponent, and dismisses Pat's steaks. (So why do so many people eat them? "You can acquire a taste for bad food," he says.) Olivieri, who went to a Quaker high school, refuses to be drawn into a shouting match. "I don't even call him a competitor," he sniffs.

Whatever. But Olivieri can't deny there's a rivalry. It's addictive, even. I ask him what he'd do if he woke up one day and Geno's was gone. "I'd feel a void--that'd be hard," Olivieri admits, then quickly adds, "I'd buy the place and open it up again. And call it Geno's. And fight with myself."