  We report experimental results for the Kelvin-Helmholtz instability between
two immiscible fluids in parallel flow in a Hele-Shaw cell. We focus our
interest on the influence of the gap size between the walls on the instability
characteristics. Experimental results show that the instability threshold, the
critical wavelength, the phase velocity and the spatial growth rate depend on
this gap size. These results are compared to both the previous two-dimensional
analysis of Gondret and Rabaud (1997) and the three-dimensional analysis
recently derived by Plourabou=E9 and Hinch (2001), showing that the agreement
is still not complete especially when gap size increases.
