The National Trust for Historic Preservation wants Charleston, South Carolina officials to study the potential downsides of tourism from cruise ships.
A first-ever warning from the group comes as Savannah plans to lure cruise ships, as well.
The National Trust is known for its annual list of endangered historic places.
For the first time in its history, however, it added a supplemental "Watch List" to the list.
It says, cruise ship tourism in Charleston could harm the city's historic character.
Daniel Carey of the Historic Savannah Foundation says, he's concerned Savannah runs the same risk, since Savannah is considering welcoming cruise ships.
He says, the city isn't studying the potential social and cultural changes that thousands of new tourists and their cars could bring.
"It's a fire bell in the night for Savannah," Carey says. "We should pay very close attention to what's going on in Charleston."
Carey says, Savannah can manage the influx with good management.
But so far, the city has studied only potential economic benefits, not potential downsides to the city's historic character.
"The economic impacts are merely one point in a number of points that need to be resolved," Carey says. "I become a little concerned when I see something that appears to be moving forward and yet we're not at the discussion table."
The National Trust is a non-profit advocacy and resource organization for historic preservation and has no legal authority over federally-designated historic districts.
Historic Savannah's status as the nation's largest National Landmark District comes from the U.S. Department of the Interior.