Natural convection in horizontal differentially heated annuli is numerically investigated both by linear stability analysis and by solving the three-dimensional flow equations for a radius ratio R = 1.7. The governing equations were solved numerically by using a finite volume method. The stability of the basic crescent-shaped flows to three-dimensional disturbances is investigated. The existence of a stability region corresponding to a reversal from 3D- to 2D-flows is predicted for the first time. Three-dimensional computations show that multiple solutions are possible in the new stability region according to the initial conditions. Computations performed at supercritical Rayleigh numbers for the oscillatory regime elucidated the influence of the annulus length. Conflicting results reported in the archival literature are clarified. (c) 2005 Elsevier Ltd. All rights reserved.