[en] We have studied the origin of the cooperative Jahn-Teller effect in PrCu₂. We have determined the anisotropic phase diagram by measuring the thermal expansion and the magnetostriction in magnetic fields up to 15 T. The Jahn-Teller transition temperature is almost constant for the field along the a- and c-axis, while it decreases with increasing the field along the b-axis. The phase diagram can be explained qualitatively by including the quadrupolar interaction for left angle O_xy right angle in the crystalline electric field Hamiltonian. (orig.)