Natural convection ventilation of a rectangular chamber containing a heated rectangular body has been investigated both experimentally and computationally. The heated square blockage within the partial enclosure was adjacent to the adiabatic lower surface of the chamber and the three exposed surfaces of the block were such as to give a constant heat flux. The air inlet and outlet were located at the bottom and top of the chamber vertical walls respectively, and were of equal area. Velocity measurements were performed using laser Doppler anemometry (LDA) with a one-component He-Ne laser connected to a burst spectrum analyser (BSA). Detailed velocity profiles were measured at the inlet, outlet and at several locations inside the chamber. Temperatures of the heated air at the outlet were measured with a chromel-alumel (type K) bare wire thermocouple probe. Three-dimensional laminar and turbulent (k-epsilon model) numerical simulations were obtained by solving the governing equations using the computational fluid dynamics (CFD) code PHOENICS. Comparisons of experimental and computational results showed very good agreement in most of the flow field.