The temperature dependence of the structure factor S(k) of the instantaneous magnetic fluctuations in copper formate tetradeuterate has been investigated using neutron scattering techniques. Our data extend to unprecedently high temperatures (T similar to J) compared to the 2D exchange coupling. The correlation length xi and amplitude S-0 are in good agreement with the predictions for a 2D S = 1/2 Heisenberg antiferromagnet on a square lattice.