  Neutron scattering measurements on the one-dimensional gapped S=1
antiferromagnet, CsNiCl3, have shown that the excitation corresponding to the
Haldane mass gap Delta at low temperatures persists as a resonant feature to
high temperatures. We find that the strong upward renormalisation of the gap
excitation, by a factor of three between 5 and 70K, is more than enough to
overcome its decreasing lifetime. We find that the gap lifetime is
substantially shorter than that predicted by the scaling theory of Damle and
Sachdev in its low temperature range of validity. The upward gap
renormalisation agrees with the non-linear sigma model at low temperatures and
even up to T of order 2Delta provided an upper mass cutoff is included.
