A solution methodology is developed to obtain three-dimensional natural convection in a honeycomb enclosure with hexagonal end walls. The numerical methodology is based on an algebraic coordinate transformation technique, which maps the complex cross section onto a rectangle, coupled with a calculation procedure for fully elliptic three-dimensional flows. To the authors' knowledge this is the first three-dimensional numerical study of the problem. The results are obtained for laminar flow and for both conduction and adiabatic side wall thermal boundary conditions. The calculations are performed for Rayleigh numbers in the range of 103to 105, for a Prandtl number of 0.7, and for five values of the aspect ratio of the honeycomb enclosure. The average Nusselt number results are compared with the corresponding values for two-dimensional rectangular enclosures.