  Recently A. Zamolodchikov obtained a series of identities for the expectation
values of the composite operator T\bar{T} constructed from the components of
the energy-momentum tensor in two-dimensional quantum field theory. We show
that if the theory is integrable the addition of a requirement of factorization
at high energies can lead to the exact determination of the generic matrix
element of this operator on the asymptotic states. The construction is
performed explicitly in the Lee-Yang model.
