  We use the three dimensional uniformly frustrated XY model, as a model for a
high temperature superconductor in an applied magnetic field, to explicitly
measure the longitudinal correlation length xi_z in the vortex line liquid
phase. We determine the scaling of xi_z with magnetic field and system
anisotropy close to the vortex lattice melting transition. We apply our results
to determine the extent of longitudinal correlations in YBCO just above
melting.
