The microscopic transport equations for free fields are solved using the Schwinger function. Thus, for general initial conditions, the evolution of the energy-momentum tensor is obtained, incorporating the quantum effects exactly. The result for relativistic fermions differs from classical hydrodynamics, which is illustrated for Landau and Bjorken type initial conditions in this model of exploding primordial matter. Free fermions behave like classical dust concerning hydrodynamic observables. However, quantum effects which are present in the initial state are preserved. PACS numbers:Often the complicated time dependent dynamics of quantum many-body systems or fields is approximated by a perfect fluid model. Since the seminal work by Fermi and Landau this approach has been applied successfully, in order to study global features, such as multiplicity distributions and apparently thermal transverse momentum spectra of produced particles, in high-energy collisions of strongly interacting matter [1,2,3,4]. Similarly, the hydrodynamic approximation is often invoked in astrophysical applications and cosmological studies of the early universe [5] Recently it has been shown that a free scalar field indeed behaves like a perfect fluid in the semiclassical (WKB) regime [6]. More generally, the mechanisms of quantum decoherence and thermalization in such systems which can be described hydrodynamically, i.e. the emergence of classical deterministic evolution from an underlying quantum field theory, are of fundamental interest [7,8,9,10].The limitations of the fluid picture, however, have rarely been explored in the microscopic or high energy density domain. Difficulties reside in the derivation of consistent transport equations and in the amount of computation required to find realistic solutions; see Refs. [11], for example, for a review and recent progress concerning selfinteracting scalar particles and the quark-gluon plasma, respectively. More understanding of related hydrodynamic behavior, if any, seems highly desirable.Presently, we study the relation between relativistic hydrodynamics and the full quantum evolution of a free matter field. In the absence of interactions, decoherence or thermalization may be present in the initial state, corresponding to an impure density matrix, but is followed by unitary evolution. We consider this as a "quantum dust" model of the expansion of matter originating from a high energy density preparation phase, which the Landau and Bjorken models describe classically [2,3].Our approach is independent of the nature of the field, as long as it obeys a standard wave equation. To be definite, we choose to work with Dirac fermions and comment about neutrinos later. We introduce the spinor Wigner function, i.e., a (4x4)-matrix depending on space-time and four-momentum coordinates: