  The exact amplitude for the asymptotic correlation function in the S=1/2
Heisenberg antiferromagnetic chain is determined: <S^a_0 S^b_r> goes to (-1)^r
delta^{ab}(ln r)^{1/2}/[(2 pi)^{3/2}r]. The behaviour of the correlation
functions for small xxz anisotropy and the form of finite-size corrections to
the correlation function are also analysed.
