Gas-solid flow in high-density circulating fluidized-bed risers differs significantly from that in conventional flow regimes, including fast fluidization and pneumatic conveying. It is shown that when there is no downward flux at the wall at high suspension densities, and hence no core/annulus structure, the flow corresponds to a dense suspension up-flow regime. This flow regime is explained, and ifs key properties given. A lower boundary is also proposed for this new flow regime based on available data.