  A massive neutrino which decays after recombination (t > 10^{13} sec) into
relativistic decay products produces an enhanced integrated Sachs-Wolfe effect,
allowing constraints to be placed on such neutrinos from present cosmic
microwave background anisotropy data. Previous treatments of this problem have
approximated the decay products as an additional component of the neutrino
background. This approach violates energy-momentum conservation, and we show
that it leads to serious errors for some neutrino masses and lifetimes. We redo
this calculation more accurately, by correctly incorporating the spatial
distribution of the decay products. For low neutrino masses and long lifetimes,
we obtain a much smaller distortion in the CMB fluctuation spectrum than have
previous treatments. We combine these new results with a recent set of CMB data
to exclude the mass and lifetime range m_h > 100 eV, \tau > 10^{12} sec. Masses
as low as 30 eV are excluded for a narrower range in lifetime.
