As more and more doctors join physician groups, antitrust experts wonder whether hospitals and health plans will be ... outgunned.

To increase market leverage, more doctors are forming networks. That's setting off alarms among economists and antitrust experts who say such organizations are mushrooming without proven benefits for consumers. Some argue that more areas of the country could be saddled with monopolies that stifle competition. Hospitals and health plans fear physicians' new power could leave them outgunned.