The results of theoretical and experimental studies of the laminar mixed convection of water flow in a horizontal isothermally cooled pipe with the density inversion effect are presented. The theoretical solution uses the large Prandtl number assumption. The governing parameters of the flow and their ranges are the Rayleigh number Ra, O less than or equal to Ra less than or equal to 5 x 10(7), and the inlet temperature T-0, 5 degrees C less than or equal to T-0 less than or equal to 18 degrees C, and wall temperature, T-w = 0 degrees C. The effects of the density inversion on the flow and temperature fields and the Nusselt numbers are presented. The experiments were carried out by measuring the inlet, outlet and wall temperature. The secondary flow patterns were visualized by injecting a few drops of mercurochrome solution into the inlet tube. The measured and calculated heat transfer rate as well as photographs of the visualized secondary flow patterns and the numerically predicted flow patterns with density inversion effect are also presented.