This paper presents experimental and numerical results for natural convection in a horizontal, annular cavity which communicates with the surroundings through its open end. In the experimental study, a known heat flux was applied to each component of the cavity (inner cylinder, inner cylinder tip, outer cylinder and end-wall) and local surface temperature measurements were made to determine heat transfer characteristics of the convective flow. Trends in the experimental data have been explained in terms of the physical mechanisms underlying the buoyancy induced flow. Smoke flow visualization using laser-induced lighting was performed to understand the flow field around. the open end of the cavity. The heat transfer results were correlated by Nu(av) = 0.0131(Ra*)(0.378) for the range of Rayleigh numbers considered (1.3 x 10(9) < Ra* < 5.1 x 10(9)) in the experiments. In the numerical investigation, solutions to the three-dimensional time-averaged (Reynolds’) steady-slate equations of fluid motion and heat transfer, were obtained using a finite element analysis. Results of the conjugate study including the local temperature distributions, heat transfer coefficients and the flow field showing the interactions between the ambient and cavity flow fields agreed favorably with experimental results. The present work provides, for the first time, validated heat transfer data for high Rayleigh number buoyancy-induced flows in open annular cavities.