Direct comparisons between the results of a numerical model and laboratory experiments are presented for density-stratified flows around an isolated three-dimensional hill. The numerical model integrates the Navier-Stokes equations for incompressible stratified flow using a finite-difference scheme. Numerical calculations were made for Reynolds numbers (based on the hill height) of 100 and 400 and for Froude numbers between 0.5 and 2 as well as for neutrally stratified flow. The laboratory experiments were performed in a small (2.1m long) stratified towing tank, a large (25m long) stratified towing tank, and a meteorological wind tunnel. Experimental results are presented for Reynolds numbers between 100 and 40,000 and for the same Froude numbers as used in the numerical calculations. The numerical model with a Reynolds number of 100 compared best with the large scale laboratory experiments, although there were substantial differences near the hill surface and in the wake of the hill.