TORONTO — For all the concern about the gentrification, rising housing prices and the growing gap between the rich and poor in our leading cities, an even bigger threat lies on the horizon: The urban revival that swept across America over the past decade or two may be in danger. As it turns out, the much-ballyhooed new age of the city might be giving way to a great urban stall-out.

Starting around the turn of the millennium, young, affluent professionals began pouring into the cores of big cities, reversing generations of white flight. Unlike their parents and grandparents, these new urbanites embraced the energy and authenticity — and the ethnic, racial and sexual diversity — that are emblematic of cities. Established corporations and high-flying tech start-ups followed suit. The urban revival has been so thoroughgoing that it has even engendered a new crisis of success, whose symptoms are runaway gentrification, soaring housing prices and a widening income gap between newcomers and longtime residents.

But even as people and companies continue to pour into cities, signs that the tide has crested are emerging.

While many, if not most, large cities grew faster than their suburbs between 2000 and 2015, in the last two years the suburbs outgrew cities in two-thirds of America’s large metropolitan areas, according to a detailed analysis of the latest census data by the demographer William Frey of the Brookings Institution. Fourteen big cities lost population in 2015-16 compared with just five in 2011-12, with Chicago, the nation’s third-largest city, hemorrhaging the most people.