  We present the first results of a program to systematically study the
optical-to-X-ray spectral energy distribution (SED) of Swift GRB afterglows
with known redshift. The goal is to study the properties of the GRB explosion
and of the intervening absorbing material. In this report we present the
preliminary analysis on 23 afterglows. Thanks to Swift, we could build the SED
at early times after the GRB (minutes to hours). We derived the Hydrogen column
densities and the spectral slopes from the X-ray spectrum. We then constrained
the visual extinction by requiring that the combined optical/X-ray SED is due
to synchrotron, namely either a single power law or a broken power law with a
slope change by 0.5. We confirm a low dust-to-metal ratio, smaller than in the
SMC, even from the analysis of data taken significantly earlier than previously
possible. Our analysis does not support the existence of ``grey'' dust. We also
find that the synchrotron spectrum works remarkably well to explain afterglow
SEDs. We clearly see, however, that during the X-ray steep decay phases and the
flares, the X-ray radiation cannot be due only to afterglow emission.
