The influence of free convection on forced laminar flow of water initially at the ice point was investigated. The flow was horizontal in a circular tube with uniform wall heat flux and a fully developed velocity profile at the onset of heating. Local measurements were made for the calculation of Nusselt number, Reynolds number, Prandtl number, and Grashof number. Reynolds numbers ranged from 120 to 2300, Grashof numbers ranged up to 2.5 × 105, and Graetz numbers ranged from 1.5 to 1000. The Nusselt number was not significantly affected in the thermal entry region, but values which are 2 1/2 times those expected for uniform properties were discovered far down the tube at x/D ≈ 700. The data were correlated with fair agreement by the parameter (GrPr)1/4/NuGz, where NuGz is the Nusselt number based on locally uniform physical properties.