The association of a pair of hydrophobic solutes in water has been investigated by free energy molecular dynamics simulations of a system containing 516 water molecules. Convergence of the calculations is guaranteed by the comparison of data obtained with two independent free energy sampling techniques, which have been optimized for our system. Coulomb interactions have been treated with the Ewald method. Using this computationally expensive approach many of the previously reported discrepancies in the temperature, pressure and interaction parameter dependence of hydrophobic association are clarified. A temperature effect on both the free energy of association and the equilibrium between contact and solvent-separated species is observed. Raising temperature favors association. The most pronounced temperature dependence occurs in the interval between 300 and 350K.