We develop a kinetic model for polariton condensation, in which the field dynamics of collective excitations is treated self-consistently during the condensation kinetics. This model allows to account properly for the quantum fluctuations of the condensate. Using realistic parameters, we prove that long-wavelength excitations considerably deplete the condensate just above threshold. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.