In this Letter, we describe a method for retrieving the phase of a wavefield from a volume diffraction pattern. We show at first that the magnitude of the 3D Fourier transform of a diffracted volume wavefield is concentrated around a paraboloid. For the phase retrieval, we apply iteratively the constraints of the measured intensity and the paraboloid (sparsity) constraint in the 3D Fourier domain. Experimental validations and comparisons to other methods are presented.