  We present Chandra observations of 18 spectroscopically selected quasars,
already known to be X-ray weak from previous ROSAT observations. All the
sources but one are detected by Chandra, and spectral analysis suggests that
most of them are intrinsically underluminous in the X-rays (by a factor from 3
to >100). These objects could represent a large population of quasars with a
Spectral Energy Distribution different from that of standard blue quasars. We
discuss the possibility that a significant fraction of the obscured AGN needed
in Synthesis models of the X-ray background could be instead optically
broad-line, X-ray weak quasars.
