Antitrust regulators face a tough decision on whether to allow Google to buy ITA Software, a company that organizes online flight information.

Once it gets into that business, Google may well figure out brilliant new ways to help travelers organize and book vacations online. But Google’s dominance of online search  which allows it to steer users to certain Web sites and away from others  raises a real concern about the potential consequences of the deal.

Google cannot abuse its dominance in search to shut out the competition. What would happen to the $80 billion-a-year online travel business if Google’s rivals were relegated to the nether reaches of its search results and it came to dominate the search for online tickets, too?

This concern comes up every time Google adds a new service  comparison shopping, maps, a bookstore. AOL’s Mapquest was clobbered when Google entered that business, dropping from first place to a distant second in less than two years. This was partly because of Google’s putting its own maps on top in response to queries about locations. In November, the European Commission opened an investigation into whether Google was illegally promoting its services and penalizing rivals.