  The validity of the semiclassical approximation is investigated during the
preheating phase in models of chaotic inflation using a modification of a
criterion previously proposed for semiclassical gravity. If the modified
criterion is violated then fluctuations of the two-point function for the
quantum fields are large and the semiclassical approximation is not valid.
Evidence is provided that the semiclassical approximation breaks down during
the early stages of preheating, well before either scattering effects or
backreaction effects are important.
