  We derive an expression for the luminosity distance in FLRW spacetimes
affected by scalar perturbations. Our expression is complete to linear order
and is expressed entirely in terms of standard cosmological parameters and
observational quantities. We illustrate the result by calculating the RMS
scatter in the usual luminosity distance in flat (Omega_m,Omega_Lambda) =
(1.0,0.0) and (0.3,0.7) cosmologies. In both cases the scatter is appreciable
at high redshifts, and rises above 11% at z = 2, where it may be the dominant
noise term in the Hubble diagram based on SN Ia.
