Steady-state laminar natural convection in a cubical enclosure with a cold Vertical wall and a hot square sector on the opposite wall is numerically studied. The flow pattern consists of a single, symmetric circulation cell. The transition from conduction to convection regime ends at Ra=10(5), and is characterized by conduction suppression and slow development of convection. In the range of Ra=10(5)-10(7), lateral velocities become very large, producing a highly mixed, thermally stratified three-dimensional flow, and the circulation cell undergoes cross-sectional changes. In the whole range of Ra, the Nusselt number is reduced compared to the case in which the full side wall is hot (test case).