  We estimate large-scale curvature perturbations from isocurvature
fluctuations in the waterfall field during hybrid inflation, in addition to the
usual inflaton field perturbations. The tachyonic instability at the end of
inflation leads to an explosive growth of super-Hubble scale perturbations, but
they retain the steep blue spectrum characteristic of vacuum fluctuations in a
massive field during inflation. The power spectrum thus peaks around the
Hubble-horizon scale at the end of inflation. We extend the usual delta-N
formalism to include the essential role of these small fluctuations when
estimating the large-scale curvature perturbation. The resulting curvature
perturbation due to fluctuations in the waterfall field is second-order and the
spectrum is expected to be of order 10^{-54} on cosmological scales.
