The effects of the hydrodynamic interactions on the short-time dynamics of
colloidal, hard-sphere-like particles confined between two parallel walls a
re measured by digital videomicroscopy. We find that such effects can be de
scribed in terms of an effective two-dimensional hydrodynamic function H(k)
, de fined as a straightforward adaptation to two dimensions of the corresp
onding object describing collective dynamics for the three-dimensional (3D)
suspensions. Interestingly, the behavior of H(k) is qualitatively similar
to the hydrodynamic function of 3D suspensions of hard spheres. We also fou
nd that for values of k where the static structure factor is 1, the dynamic
s is determined only by self-diffusion.